Design and synthesis of novel zeotypes

Abstract

This thesis combines synthetic and structural studies, alongside molecular modelling calculations to design and prepare novel zeotypes (aluminophosphates/ AlPOs, silicoaluminophosphates/ SAPOs etc) as potential catalysts for emission control in collaboration with Johnson Matthey and following on from the work done by A. Turrina.

Underpinning most of the work reported in this thesis are structural studies, for example Rietveld refinement against PXRD data. This method within a multi-technique approach, led to the structure solution of as-prepared SAPO STA-20 (SWY). It was determined that trimethylamine, one of the organic structure directing agents (OSDAs) used in a co-templated synthesis with diDABCO-C6, is present in both the gme and swy cages within the STA-20 framework. Full details are given in Chapter 4.

There are several ways to prepare zeotype catalysts. In Chapter 4, the use of new templates for existing materials (SAPO-56, SAPO STA-18 and AlPO-17) with the aim of improving properties is discussed. The synthesis of SAPO-56 (AFX) and STA-18 (SFW) has been achieved using the OSDAs hexamethonium and decamethonium bromide respectively, in each case with trimethylamine in a co-templating strategy. The results extend the previous work of Turrina, who synthesised SAPO-56 and SAPO STA-18 using diDABCO-C4 and diDABCO-C6, respectively. AlPO-17 (ERI) was successfully prepared using a novel SDA, propyltrimethylammonium DABCO, which was synthesised at St Andrews. Computational modelling was used to provide starting models for structural refinement and in the targeted design of the novel hypothetical zeotype, SAPO DH-3. This route of preparing zeotype catalysts is more challenging but it has good potential in enhancing catalytic properties for specific applications. Through attempts of preparing DH-3, another new template for SAPO-56 was found.

Another route of preparing zeotype catalysts is to intergrow two existing materials together. Intergrowths are expected to have improved properties and should be patentable. In Chapter 5, the synthesis of a series of different intergrowths belonging to the ABC-6 family is explored. A SAPO-56/SAPO-34 (AFX/CHA) intergrown material, denoted STA-23 (synthesised using a triple template approach), was further investigated and the methodology introduced into the SAPO STA-18 synthesis to produce a new disordered material denoted STA-24. In these syntheses, copper polyamine complexes were included in the one-pot synthesis. The Cu²⁺ cations can then be released to extra framework positions upon calcination. The Cu SAPOs have potential application in the selective catalytic reduction of NOx by ammonia for emission control technology. STA-23 and STA-24 are believed to be faulted structures due to peak broadening effects observed in the PXRD patterns. Preliminary DIFFaX simulations to model layer disorder were performed on the STA-23 system (a model for the STA-24 materials is currently being developed) and shows that SAPO-56 has a 20% faulting probability with SAPO-34 and an additional faulted SAPO-34 phase present. The latter part of Chapter 5 discusses the synthesis of ‘Cu-free’ intergrowths for potential use in methanol-to-olefin catalysis. A SAPO STA-20/SAPO-17 faulted material, given the name ‘SWY/ERI’ was prepared successfully using the triple template approach although further work is needed in order to confirm that the material is indeed an intergrowth of STA-20 and SAPO-17.

Chapter 6 describes the synthesis and optimisation of the novel AlPO, STA-28. This material was prepared using 1,10-phenanthroline as the template. Molecular modelling suggested that phenanthroline is a suitable OSDA for the larger cage found within the SAPOs STA-6 and STA-7 (so to be used in place of cyclam, which although a very effective template in this case, is expensive making the preparation of these materials on an industrial scale unsustainable). An unknown phase resulted from exploratory syntheses using phenanthroline, in which the crystallites were of sufficient quality and size to be solved by single-crystal X-ray diffraction methods. The new material was subsequently denoted ‘STA-28’ and has the remarkable feature that the phenanthroline template is bound to the framework Al, giving an octahedral Al site as confirmed by ²⁷Al solid-state magic-angle spinning NMR (nuclear magnetic resonance) spectrum. Upon calcination, the geometry of this particular Al site changes from 6-fold to 4-fold and the predicted lattice energy of the ‘empty’ framework exceeds values reported for known tetrahedral AlPOs. This suggests that the tetrahedral framework cannot be synthesised directly but instead via an inorganic-organic hybrid.

Chapter 7 explores the crystal chemistry of AlPO STA-28. It was found that other phenanthroline analogues, for example, 4,7-dimethyl-1,10-phenanthroline, can be used in the synthesis of STA-28. Phenanthroline can also be used to direct specific metal substitution to the octahedral Al site, with Mg²⁺, Fe³⁺ and Sc³⁺ all being shown to substitute for the 6-fold site only by structural refinement and NMR methods. The scope of this work was also extended to the use of 2,2-bipyridyl in the STA-28 synthesis. However, this instead produced an unknown phase with work currently ongoing in order to solve its structure.

Date of Award	26 Jun 2019
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Paul Wright (Supervisor), John L. Casci (Supervisor) & Alessandro Turrina (Supervisor)