Novel zeotype frameworks for catalytic applications

: design, synthesis and characterisation

Abstract

A combination of experimental synthetic and structural studies, performed in the light of modelling, has been used to design and prepare novel microporous silicoaluminophosphate (SAPO) zeotypes of potential application as catalysts.

Cu²⁺ and Ni²⁺ cations complexed by linear polyamines have been studied as structure- directing agents (SDAs) for the direct synthesis of copper- and nickel-containing microporous silicoaluminophosphate (SAPO) materials. The complexing ligands diethylenetriamine (DETA), N-(2-hydroxyethyl)ethylenediamine (HEEDA), triethylenetetramine (TETA), N,N′-bis(2-aminoethyl)-1,3-propanediamine (232), 1,2- bis(3-aminopropylamino)ethane (323), tetraethylenepentamine (TEPA) and pentaethylenehexamine (PEHA) have been investigated. Cu and Ni forms of both SAPO-18 and SAPO-34 materials have been prepared. While most Cu and Ni polyamine complexes direct crystallisation to SAPO-34, SAPO-18 has been prepared with Cu²⁺ 232 and Ni²⁺ complexes of 232 and TETA. The coordination geometry of the included metal complexes was studied by UV-visible and EPR spectroscopy and computer simulation. SAPO-18 is favoured by the smaller square planar complexes or octahedral species (with 2 water molecules) of 232 and TETA. Rietveld analysis has allowed to locate extra-framework Cu²⁺ and Ni²⁺ cations released by calcination. In SAPO-34, Cu²⁺ and Ni²⁺ were located within the cha cage within the 8R window. A second site was found for Ni²⁺ at the centre of the six-membered rings (6Rs) of the double-six-ring (D6R) sub- units. In SAPO-18 both Cu²⁺ and Ni²⁺ cations were located only in the 6Rs of the D6R sub-units. Selected copper SAPO-18 and SAPO-34 samples were tested in the selective catalytic reduction of NO with ammonia (NH₃-SCR): both showed high activity.

SAPO-34-containing Fe cations in extra-framework position has been prepared by the one-pot approach for the first time using DETA, TETA, TEPA and PEHA as SDAs. The coordination sphere and oxidation state of the Fe cations within the as-prepared and calcined SAPO-34 materials specifically those prepared with Fe-TEPA, were investigated by combining UV-visible and Mössbauer spectroscopies. Iron cations were found to be in two different octahedral coordinations within the as-prepared hydrated SAPO-34: assigned as [Fe(TEPA)(H2O)]²⁺ and [Fe(TEPA)(O2)]²⁺, respectively. Furthermore the Fe²⁺-TEPA complex contained within SAPO-34 was found to have interesting properties as a reversible O2 carrier. The calcined sample contains octahedral isolated or dimeric Fe³⁺ species well dispersed within the SAPO-34 pores. In order to prove the novelty of our material, Fe-SAPO-34 was synthesised as described in a U.S. Patent Application 2012/0251422. Freshly calcined, one-pot synthesised, Fe- SAPO-34 material shows a greater activity for the SCR DeNOx process than that shown by the US patent sample.

A retro-synthetic method to design the synthesis of target silicoaluminophosphates whose framework belongs to the ABC-6 structural family and has at least one gme cage per unit cell has been successfully developed. This strategy allowed the preparation of SAPO versions of AFX (SAPO-56), SFW (STA-18) and GME (STA-19) types of framework. To the best of my knowledge, the last two SAPOs have been prepared for the first time. The method is based on the use of two SDAs simultaneously, to promote the formation of structure features such as cages or channels of the target framework type. Computational modelling was employed to locate energetic favourable quaternary ammonium compounds and diquaternary salts of diazabicyclooctane (DABCO) with optimal shape and size to template the gme cage and the secondary cages or channels of the target structures, respectively. Trimethylammonium cations showed the most favourable energies for the gme cage while bisDABCO salts and quaternary ammonium oligomers of DABCO with connecting polymethylene chain lengths between 4 and 8 methylene units showed a good fit for the secondary cages or channels. The incorporation and location of the SDAs were determined combining ¹³C MAS NMR, CHN, TGA, computational modelling and Rietveld refinement on PXRD data.

Trimethylamine and diDABCO-C6 have been found to template the crystallisation of a new microporous silicoaluminophosphate framework structure, denoted STA-20 (St. Andrews microporous material-20). The structure of SAPO STA-20 was determined by combining powder diffraction, electron crystallography, and computer modelling (P-31c, a = b = 13.15935(26) Å, c = 30.5837(6) Å). STA-20 is a unique member of the ABC-6 family of which the stacking sequence, AABAABAACAAC(A), is the equal longest stacking sequence in the subfamily of ABC-6 structures consisting of 6Rs and D6Rs, on a par with the SAT structure. Moreover STA-20 contains the longest cavity observed in ordered ABC-6 materials. The location of the templates within the framework was determined by combining computer modelling and Rietveld refinement against X-ray data. MAS NMR and N₂ adsorption have also been used to further characterise the solid.

Two new disordered silicoaluminophosphate structures belonging to the ABC-6 family were obtained by the combination of specific SDAs. GME-CHA and babelite-AFX intergrowth structures, generically named SAPO STA-22, were prepared by combining trimethylamine, cyclam and ferrous acetate in a SAPO preparation. An intergrowth AFX-CHA structure, denoted SAPO STA-23, was synthesised by partially replacing diDABCO-C4 with Cu- or Fe-TEPA in the preparation of SAPO-56 employing the ‘double- template’ method mentioned above giving a transition-metal containing SAPO-56 by the one-pot approach. The selected-area electron diffraction (SAED) patterns and the simulations of the experimental PXRD pattern performed employing the computer program DIFFaX have allowed us to determine the types of stacking in the materials, which solid-state UV-visible spectroscopy, MAS NMR and N2 adsorption have been used to further characterise them.

Date of Award	30 Nov 2015
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Paul Anthony Wright (Supervisor)