Alessandro Turrina

An Aluminophosphate Molecular Sieve with 36 Crystallographically Distinct Tetrahedral Sites

The structure of the new medium-pore aluminophosphate molecular sieve PST-6 is determined by the combined use of rotation electron diffraction tomography, synchrotron X-ray powder diffraction, and computer modeling. PST-6 was prepared by calcination of another new aluminophosphate material with an unknown structure synthesized using diethylamine as a structure-directing agent, which is thought to contain bridging hydroxy groups. PST-6 has 36 crystallographically distinct tetrahedral sites in the asymmetric unit and is thus crystallographically the most complex zeolitic structure ever solved.

A Family of Molecular Sieves Containing Framework‐Bound Organic Structure‐Directing Agents

Organic structure-directing agents (OSDAs), such as quaternary ammonium cations and amines, used in the synthesis of zeolites and related crystalline microporous oxides usually end up entrapped inside the void spaces of the crystallized inorganic host lattice. But none of them is known to form direct chemical bonds to the framework of these industrially important catalysts and adsorbents. We demonstrate that ECR-40, currently regarded as a typical silicoaluminophosphate molecular sieve, constitutes instead a new family of inorganic-organic hybrid networks in which the OSDAs are covalently bonded to the inorganic framework. ECR-40 crystallization begins with the formation of an Al-OSDA complex in the liquid phase in which the Al is octahedrally coordinated. This unit is incorporated in the crystallizing ECR-40. Subsequent removal of framework-bound OSDAs generates Al-O-Al linkages in a fully tetrahedrally coordinated framework.

Crystal form selectivity by humidity control: the case of the ionic co-crystals of nicotinamide and CaCl2

Post-synthesis (de)hydration techniques were used here to explore further hydrated forms of ionic co-crystals (ICCs) of nicotinamide with CaCl2. Humidity is shown to be a crucial factor for ICCs, which cannot be ignored for a complete polymorph screening of this class of compounds. The exposure of nicotinamide·CaCl2·H2O obtained by kneading reaction to a controlled relative humidity of 75% led to the formation of a new hydrated phase: nicotinamide·CaCl2·4H2O. When nicotinamide·CaCl2·H2O was exposed to a relative humidity between 32% and 54%, nicotinamide2·CaCl2·2H2O was obtained. The anhydrous form was achieved as the result of overnight dehydration of nicotinamide·CaCl2·H2O at 150 °C. The tetrahydrate form cannot be obtained as a first product for kinetic (or thermodynamic) reasons. Control of the relative humidity has proven to be an efficient way to selectively isolate and stabilize powders of pure hydrous ICC phases, which is fundamental for industrial applications. Crystalline structures of nicotinamide·CaCl2·4H2O and anhydrous nicotinamide·CaCl2 were determined by powder diffraction.

Synthesis and activation for catalysis of Fe-SAPO-34 prepared using iron polyamine complexes as structure directing agents

The use of transition metal cations complexed by polyamines as structure directing agents (SDAs) for silicoaluminophosphate (SAPO) zeotypes provides a route, via removal of the organic by calcination, to microporous solids with well-distributed, catalytically-active extra-framework cations and avoids the need for post-synthesis aqueous cation exchange. Iron(II) complexed with tetraethylenepentamine (TEPA) is found to be an effective SDA for SAPO-34, giving as-prepared solids where Fe2+–TEPA complexes reside within the cha cages, as indicated by Mossbauer, optical and X-ray absorption near edge spectroscopies. By contrast, when non-coordinating tetraethylammonium ions are used as the SDAs in Fe-SAPO-34 preparations, iron is included as octahedral Fe3+ within the framework. The complex-containing Fe-SAPO-34(TEPA) materials give a characteristic visible absorption band at 550 nm (and purple colouration) when dried in air that is attributed to oxygen chemisorption. Some other Fe2+ polyamine complexes (diethylenetriamine, triethylenetetramine and pentaethylenehexamine) show similar behaviour. After calcination in flowing oxygen at 550 °C, ‘one-pot’ Fe(TEPA) materials possess Fe3+ cations and a characteristic UV-visible spectrum: they also show appreciable activity in the selective catalytic reduction of NO with NH3.

Molecular Modelling of Structure Direction Phenomena

Organic structure-directing agents (OSDAs) are widely used in the synthesis of zeolitic materials. Molecular modelling methods are playing a key part in helping to establish the role of the OSDA in the synthesis process. Moreover, modelling is increasingly being used to design and screen new OSDAs for specific targets. This review aims to provide an overview of the methods used to investigate the relationship between OSDAs and their zeolitic products and to provide a series of examples to highlight the important contribution that modelling is making in this field.

Metal Complexes as Structure-Directing Agents for Zeolites and Related Microporous Materials

Metal complexes can act as structure-directing agents (SDAs) for zeolites and zeotypes, either alone or together with additional SDAs in dual-templating approaches. Such complexes include organometallic cobaltocenium ions, alkali metal crown ether complexes, first-row transition-metal (Fe, Co, Ni, Cu) polyamines and thiol-complexed second- and third-row transition metals (Pd, Pt). Their inclusion has been demonstrated in some cases by crystallographic methods but more commonly by spectroscopy (UV-visible, X-ray absorption, Mossbauer). The unique feature of this class of template is that they can not only direct crystallisation but also give solids with homogeneously distributed metal cations or metal oxide species upon calcination, precluding the need for an additional post-synthesis modification step. Materials prepared via this ‘one-pot’ synthetic route have been shown to give shape-selective catalysts for reactions such as the selective catalytic reduction of NO x with ammonia and the hydrogenation, dehydration and oxidative dehydrogenation of small hydrocarbons and oxygenates.

Further studies on ionic co-crystals: dehydration and hydration behaviour

a Department of Earth Sciences, University of Cambridge, Downing St, Cambridge, CB2 3EQ, United Kingdom b Departament of Chemistry “G. Ciamician”, University of Bologna, Via Selmi 2, 40126, Bologna, Italy c School of Chemistry, University of St Andrews, St Andrews, North Haugh, Fife, KY16 9ST, United Kingdom d Department of Chemistry I.F.M., University of Turin, via Giuria 7, Turin, 10125, Italy E-mail: gil21@cam.ac.uk