Rosemarie Szostak

Characteristics of the synthetic heulandite-clinoptilolite family of zeolites

Heulandite-clinoptilolite zeolites can be synthesized over the complete range of Si/Al ratio (2.5 to 6) found for the natural minerals. The unit cell parameters, framework infrared vibrational characteristics and thermal properties are similar to those of their natural counterparts. Results of environmental studies involving the removal of toxic metal ions such as lead, strontium and cesium from solution show that though natural clinoptilolite efficiently removes these ions from solution, the higher aluminum content of heulandite results in an improvement in the capacity of this topology for divalent cations. The ability to readily prepare synthetic heulandite and clinoptilolite using environmentally benign methods is expected to lead to improvements in the use of this family of zeolites for similar environmental applications.

Metal substituted aluminophosphate molecular sieves as phenol hydroxylation catalysts

Abstract Substitution of transition metals for either aluminum and/or phosphorus in the AIPO 4 -11 framework is found to afford novel heterogeneous catalysts for liquid phase hydroxylation of phenol with hydrogen peroxide. AlPO 4 -11 is more active than SAPO-11 and MgAPO-11 for phenol conversion to hydroquinone. Substitution of transition metal cations, such as Fe, Co and Mn significantly improves the conversion of phenol. The activity follows the order of FeAPO-11 > CoAPO-11 > FeMnAPO-11 > MnAPO-11 ≫ AlPO 4 -11. FeAPO-11, FeMnAPO-11 and CoAPO-11 give similar product selectivities of about 1:1 hydroquinone (HQ) to catechol (CT) whereas MnAPO-11 favors the production of catechol. FeAPO-11 show comparable performance to TS-1 (titanium silicate with MFI topology) for phenol conversion, with TS-1 giving higher selectivities toward hydroquinone. Medium pore size CoAPO-11 was more active than larger pore CoAPO-50, -36 and -5. The external surfaces of the catalysts play a significant role in these oxidation reactions.

Crystallization of MFI and MEL zeolites from clear solutions

Pure MFI and MEL zeolites were obtained at 170°C from clear solutions of general composition SiO 2 : 0.0004 Al 2 O 3 : 0.30 Na[in2]O: aTAABr (or TAAOH): 40H 2 O with a = 0.03 –0.16 and TAA = TEA, TPA or TBA. The physicochemical characteristics (XRD, SEM, d.t.g., d.t.a., FT i.r., and multin.m.r.) allow one to compare the products obtained from the clear solution with those obtained from a hydrogel.

Role of alkali-metal cations and seeds in the synthesis of silica-rich heulandite-type zeolites

Clinoptilolite, the silica-rich member (Si/Al>4) of the heulandite family of zeolites, crystallizes from pure Li, Na, K, and Rb ion-containing gel systems as well as mixed Li,K, Na,K and K,Rb gels. Crystallization occurs at temperatures between 140 and 190 °C and is relatively insensitive to the nature of the silica or alumina source. Members of this family are formed over a narrow range of gel Si/Al ratio (2.5–6) and OH/SiO2 ratio (0.3–0.4 in the Na,K system and 0.6–0.9 in the Li,K system). The nature of the alkali-metal cation does not have a critical structure determining role in the synthesis but does contribute to other properties of the material including the rate of crystallization, the Si/Al ratio of the resulting crystals, the crystal size and the morphology. Potassium ions greatly increase the rate of crystallization and decrease the nucleation time. The addition of other cations to the potassium ion-containing gels slows the rate of crystallization but increases the stability of the resulting clinoptilolite crystals in the mother liquor. Sodium ions increase the Si/Al ratio of the crystals while lithium ions increase their aluminium content. Seeds (1–10 mass%) promote crystallization in the Li-, Na-, Rb- and Rb,K- containing systems, but are not necessary in gels containing K, Na,K or Li,K. In the absence of seeds, other phases co-exist or are preferred, including mordenite, phillipsite, and analcime depending on the Si/Al ratio of the gel. Crystallization time is the key parameter in preparing high purity clinoptilolite materials.

Hydrothermal synthesis of alkali cation heulandite aluminosilicate molecular sieves

Heulandite molecular sieves with a Si/Al ratio of 3.2 ~ 3.8 have been synthesized hydrothermally in the presence of the alkali cations Li + ; Li + , K + ; and Na + , K + at 190 °C at autogenous pressure. They have been characterized by X-ray powder diffraction and scanning electron micrographs. Heulandites can be formed in the Li + and Na + , K + cation systems only with ~10 wt% seed crystals. But heulandite can be synthesized in the presence of Li + , K + cations without seed crystals. The formation of heulandite-type zeolites occurs over a narrow crystallization field and depends on the Si/Al ratio, the OH − /Si ratio, and on the presence of seed crystals. Scanning electron micrographs show a thin-plate topology for the crystals. Heulandite is thermally unstable for calcination beyond 350 ~ 450 °C.

Synthesis of Molecular Sieve Phosphates

The chemistry of phosphate complexation in inorganic systems is important in agriculture and soil science and more recently in remediation technology because of the low solubility product of many of the metal phosphate salts. A large number of inorganic phosphate phases have been identified [1, 2]. In the area of catalysis the aluminophosphates have been investigated both as catalysts and catalyst supports due to their surface acidity and thermal stability. Researchers searching for zeolite-like materials in the late 1970s turned to the phosphates in their quest for new compositions with potentially novel structures. The phosphate-based molecular sieves proved to be an area rich with new compositions having topologies and building units not previously imagined [3, 4]. This chapter will cover the synthetic advances in the area of new microporous molecular sieves with an update of developments in the aluminophosphate systems.

Molecular Sieves for Use in Catalysis

Zeolites and molecular sieves are finding applications in many areas of catalysis, generating intense interest in these materials in industrial and academic laboratories. As catalysts, zeolites exhibit appreciable acid activity with shape-selective features not available in the compositionally equivalent amorphous catalysts. In addition, these materials can act as supports for numerous catalytically active metals. Major advances have occurred in the synthesis of molecular sieve materials since the initial discovery of the synthetic zeolite molecular sieve types A, X, and Y, and a great number of techniques have evolved for identifying and characterizing these materials. Added to an extensive and ever growing list of aluminosilicate zeolites are molecular sieves containing other elemental compositions. These materials differ in their catalytic activity relative to the aluminosilicate zeolites and may have potential in customizing or tailoring the molecular sieve catalyst for specific applications. Elements isoelectronic with Al+3 or Si+4 have been proposed to substitute into the framework lattice during synthesis. These include B+3, Ga+3, Fe+3, and Cr+3 substituting for Al+3, and Ge+4 and Ti+4 for Si+4. The incorporation of transition elements such as Fe+3 for framework Al+3 positions modifies the acid activity and, in addition, provides a novel means of obtaining high dispersions of these metals within the constrained pores of industrially interesting catalyst materials.

Hydrothermal Zeolite Synthesis

The diversity of structure among the zeolite frameworks surpasses that of other three-dimensional networks with 60 different topologies and numerous variations within a given topology. The number of possible novel zeolites thus is great. Today, over 150 zeolite structures have been synthesized, some of them counterparts to the naturally occurring zeolites, whereas others have no natural analog. Theoretical studies of zeolite structures and structure types indicate that only a small fraction of the configurations possible for polymeric aluminosilicates have been prepared. Apparently, the major roadblock in tailoring and utilizing zeolite materials for specific catalytic and adsorbent applications is the development of synthesis methods to produce the desired composition and structure. As complicated and obscure as some of the zeolite recipes appear to the newcomer in zeolite synthesis, each component does contribute in some specific way to the success of crystallizing these materials. This chapter deals with the “ingredients” and other synthesis parameters that have a major influence in the crystallization of these microporous materials.

Identification of Molecular Sieve Structures

When we ask what characterization techniques would constitute the “bare necessities” for identification of a molecular sieve structure, two techniques stand out over the rest: X-ray powder diffraction and adsorption capacity measurements. X-ray powder diffraction indicates uniqueness in structure, as the powder diffraction pattern is a fingerprint of individual zeolite structures. The powder pattern also can provide information on the degree of crystallinity as well as phase purity, which is used for quality control in preparing different batches of known zeolite materials. Unit cell volume, calculated from the diffraction data, is sensitive to the amount of aluminum in the structural sites, expanding with decreasing SiO2/Al2O3and providing compositional information about the structure. Cell volume changes also indicate the degree of incorporation of other framework elements. Table 5.1 summarizes the information obtainable from X-ray powder diffraction.

Process of Zeolite Formation on a Molecular Level

Since zeolites were first successfully crystallized in the laboratory over 30 years ago, researchers have been trying to understand how these micropor-ous materials form from complex mixtures. Based on (1) the large number of variables that affect the crystallization process, (2) the complex chemistry of basic solutions of the amphoteric oxides of silicon and aluminum, and (3) the inhomogeneity of the crystallizing system, identification of the precursors formed on a “molecular” level—from the time of mixing of all the components until the point at which rapid crystallization (characteristic of these systems) is observable on a macroscopic scale—becomes a horrendous task. The ultimate goal is the same on both the macroscopic scale (as discussed in the previous chapter) and the microscopic scale (which will be presented here): to be able to custom-synthesize catalysts and adsorbents with structural features desirable for a given process. In considering the mechanism(s) of zeolite formation, it is essential to understand that the synthesis of zeolites is a crystallization process governed by a set of rules that differ from those applied to chemical reactions.