Synthesis and Characterization of Nanocrystalline Ferrierite Zeolite and Its Catalytic Activity

Abstract

Ferrierite (framework type FER) is a medium-pore, high-silica zeolite that contains a two-dimensional pore system consisting of 10-ring (4.2 × 5.4 Å) channels intersected by 8-ring (3.5 × 4.8 Å) channels. This zeolite is well known for the exceptional selectivity in the skeletal isomerization of n-butenes to isobutene. While ferrierite is a rare natural zeolite it can be synthesized using a variety of relatively small organic structure-directing agents (SDAs) in the presence or absence of inorganic cations. However, there is little known on the synthesis of ferrierite nanocrystals. Here we describe our attempts to synthesize the nanocrystalline ferrierite zeolite using choline ((2-hydroxyethyl)trimethylammonium, Ch+) ion, which is one of the cheapest alkylammonium ions. Furthermore, the physicochemical properties of nanocrystalline ferrierite have been characterized by various analytical tools and the catalytic properties of its proton form for the skeletal isomerization of 1-butene to isobutene have been compared with those obtained from the corresponding cation form of the commercial, microcrystalline ferrierite.The chemical composition of the synthesis mixture was 4ChOH•MCl•xAl2O3•5SiO2•150H2O for alkali metal cation conditions and 4ChOH•0.5M(NO3)2•xAl2O3•5SiO2•150H2O for alkali earth metal cation conditions, where M is Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, and Ba2+ and x is 0, 0.125, 0.25, or 0.5, respectively. The final synthesis mixture was heated at 150 °C under rotation (60 rpm) for 14 - 28 days. The synthesized nanocrystalline and the microstalline ferrierite obtained from Tosoh are characterized by using various analytical tools including powder XRD, SEM, TEM, N2 sorption, 27Al and 29Si MAS NMR, NH3-TPD, and pyridine adsorption IR. A conventional continuous-flow microreactor was used to carry out the 1-butene skeletal isomerizaton at atmospheric pressure. A reactant stream with N2/1-butene molar ratio of 9.0 was fed into a quartz reactor containing 0.1 g of zeolite catalyst at 400 °C.The overall synthesis results reveal that total four different structure types of zeolites such as intergrowth zeolite ZSM-34 (OFF/ERI), ferrierite, UZM-12 (ERI), and a lamellar silicate composed of ferrierite layers can be obtained by using Ch+ ion as an organic SDA with a different alkali metal cation and Al contents (5 ≤ Si/Al ≤ ∞). The X-ray peaks of ferrierite are too broad compared to that of the conventional one. This result indicates the crystal size of this material is much smaller than that of conventional ferrierite zeolite. When compared with the SEM image of commercial ferrierite zeolite (FER(m)) which is irregular piles of thin plate with 0.5 - 1.0 µm, of particular interest is the aggregates consisting of extremely small needles (ca. 10 × 100 nm) of this as-made ferrierite zeolite (FER(n)). While the needle-like crystal of ferrierite zeolite has been reported in the literature, nanocrystalline needle-like crystal is quite unique in this Ch+-Na+ mediated ferrierite zeolite. Of particular interest is the observation that the isobutene selectivity of H-FER(n) is almost 30% higher than that of H-FER(m) from the beginning of the reaction and is almost invariable with time on stream. The higher isobutene selectivity over the aged H-FER(n) can be understood in terms of a large number of 10-ring pore mouths per unit weight due to the much smaller crystal size while the higher initial isobutene selectivity should be come from much smaller and lower strength of strong acid sites located in 10-ring pore mouths of this material which is confirmed from NH3-TPD and pyridine adsorption IR measurements. While its cobalt ion-exchanged form shows a lower NO conversion in the selective reduction of NO with methane when compared to the corresponding form of microcrystalline ferrierite zeolite.