«Structural changes of silicoaluminophosphate materials during catalytic reactions Dissertation for degree of Philosophiae Doctor Mahsa Zokaie ...»
Structural changes of
during catalytic reactions
Dissertation for degree of
Department of Chemistry
InGAP-Innovative Natural Gas Processes and Products
Faculty of Mathematics and Natural Sciences
UNIVERSITY OF OSLO
© Mahsa Zokaie, 2012
Series of dissertations submitted to the
Faculty of Mathematics and Natural Sciences, University of Oslo
No. 1259 ISSN 1501-7710 All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission.
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Table of Contents List of publications
The author’s contribution
1.1. Zeolite and silicoaluminophosphate (SAPO) materials
1.2. Silicon island formation in SAPOs
1.3. Acidity in SAPOs
1.3.1. Brønsted acidity
1.3.2. Lewis acidity
1.4. The CHA topology
1.4. 1.1 Applications
Catalyst in methanol to olefins (MTO) conversion
1.5. SAPO-34 synthesis
1.6. Defects in crystals
2. Experimental methods
2.2.1. Powder X-ray diffraction
2.2.2. Scanning electron microscopy (SEM)
2.2.3. Energy-dispersive X-ray spectroscopy (EDS)
2.2.5. Surface area measurements
2.3. Catalytic testing and analysis of the deactivated material
3. Computational methods
3.1. Molecular mechanics
General utility lattice program (GULP)
3.2. Molecular docking simulations of the zeolite/guest system
Monte Carlo Method
3.3. Modelling of defects
3.4. Quantum Mechanical Modelling
Density Functional Theory (DFT)
4. Synopsis of results
Paper I: A computational study on heteroatom distribution in zeotype materials........... 30 Paper II: Stabilization of silicon islands in SAPOs by proton redistribution
Paper III: Unit cell expansion upon coke formation in a SAPO-34 catalyst: A combined experimental and computational study
Manuscript I: Silicon islands in SAPO materials: thermodynamic considerations from atomistic modelling (Preliminary)
Unpublished results I: Analysis of 29Si NMR spectra for varieties of shapes and sizes of silicon islands
Unpublished results II: Synthesis and characterization of SAPO-34 using N,N,Ntrimethyl-1-adamantammonium as a template
4.1 Conclusions and suggestions for future work
This thesis is mainly based on the following manuscripts and unpublished results which all are summarized in Chapter 4. The manuscripts are collected in the appendix. The contribution of the author in each manuscript is specified on page v.
Paper I: A computational study on heteroatom distribution in zeotype materials Mahsa Zokaie, Unni Olsbye, Karl Petter Lillerud and Ole Swang Microporous Mesoporous Mater.,2012, 158, 175-179 Paper II: Stabilization of silicon islands in SAPOs by proton redistribution Mahsa Zokaie, Unni Olsbye, Karl Petter Lillerud and Ole Swang J. Phys. Chem. C, 2012, 116 (13), 7255–7259 Paper III: Unit cell expansion upon coke formation in a SAPO-34 catalyst: A combined experimental and computational study Mahsa Zokaie, David Wragg, Arne Grønvold, Terje Fuglerud, Jasmina Hafizovic Cavka, Karl Petter Lillerud and Ole Swang Microporous Mesoporous Mater., 2013, 165, 1-5 Manuscript I: Lumpy Gravy: Size Distribution of Silicon Islands in SAPO Materials Based on Atomistic Modeling Mahsa Zokaie, Unni Olsbye, Karl Petter Lillerud and Ole Swang Unpublished results I: Analysis of 29Si NMR spectra for varieties of shapes and sizes of silicon islands Unpublished results II: Synthesis and characterization of SAPO-34 using N,N,N trimethyl-1adamantammonium as a template Poster I: CHA and SAPO-34: Lattice stability dependence on position of acid sites Mahsa Zokaie, Ole Swang, Stian Svelle, Merete Hellner Nilsen, Unni Olsbye and Karl Petter Lillerud ABC-6, 6th world congress on Catalysis by Acids and Bases (10-14 May 2009, Genova, Italy) Poster II: Silicon islands in SAPO materials: Thermodynamic considerations from atomistic modelling Mahsa Zokaie, Merete Hellner Nilsen, Unni Olsbye, Karl Petter Lillerud and Ole Swang 16th International Zeolite Conference (4-9 July 2010, Sorrento, Italy) Poster III: Proton Redistribution in Silicon Island of SAPO Material Mahsa Zokaie, Merete Hellner Nilsen, Unni Olsbye, Karl Petter Lillerud and Ole Swang 5th International FEZA Conference (3-7 July 2011, Valencia, Spain) The author’s contribution In Paper I, II, Manuscript I and unpublished results I, the author performed all calculations and analysis of the data. In Paper III, the author performed all experiments and calculations except for synthesis of the samples (performed by: Jasmina Hafizovic Cavka) and refinement of PXRD patterns (performed by: David Wragg). In unpublished results II, all experiments were done by the author except for FTIR measurements which were performed in the group of professor Bordiga in Turin. The author also contributed in writing and preparation of all manuscripts.
Scope The methanol-to-olefin (MTO) process is an alternative technology that can supply ethene and propene from sources other than petroleum feedstock. A range of well-known technologies that convert methanol to hydrocarbons can already be found, all of which are based on innovative catalyst systems. So far, among the materials that have been investigated for MTO catalysis, SAPO-34 has proven to be the best catalyst .The suitability of a material as a catalyst is determined by its activity, selectivity, accessibility and stability. SAPO-34 is a suitable catalyst based on the first three of these characteristics. Although the functionality of the SAPO-34 catalyst has further been demonstrated by the commercialization of the MTO process based on this catalyst , but there is still room for improvement of the current catalysts. There are both challenges due to rapid deactivation due to coke blocking the channels of the catalyst during the catalytic cycle and irreversible changes caused by reorganization of silicon during the regeneration step.
Zeolitic catalyst with the CHA topology, like SAPO-34 and SSZ-13, are among the structurally most suited materials for fundamental studies, since this is one of the few topologies where all T-sites are structurally identical. This make this material particularly suited for theoretical approaches. In this study, we addressed the effect of structural changes, both local and average, on zeolitic catalysts with the CHA toplogy. Both the effect of overall composition by comparing the aluminosilicate and SAPO version of the same structure, distribution and reorganisation of the active site are addressed as well as host guest interaction exemplified by calculations on lattice strain response to coke formation and synthesis of SAPO-34 with a new geometrically suited template.
Molecular mechanics methods have been the main method to achieve our goal, as they are well developed and have been applied in many promising studies to date. We were therefore able to focus our efforts on the chemistry of the material itself rather than developing new computational methods. In some parts of the study, experimental investigations were used to provide supplementary information to our theoretical calculations. Experimental studies comparing the different CHA type catalysts have recently been performed by other members of the Catalysis group. The calculations in this work are motivated by a urge to gain a deeper understanding of these results.
The following three chapters, entitled Introduction, Experimental and Computational Methods, present the background for present work as well as the methods that were used during the study. The last chapter summarizes the results from the study and presents the conclusions.
1.1. Zeolite and silicoaluminophosphate (SAPO) materials Zeolites are a class of crystalline microporous materials made up of [SiO4] or [AlO4] tetrahedra, which by oxygen corner sharing atoms make a three dimensional framework. These tetrahedra are the primary building blocks of the framework. The extended arrangements of linked tetrahedra with [Si-O-Si] or [Si-O-Al] sequence may be defined as secondary building units (SBUs), like three-rings, four-rings, six-rings and more complex units like double-four-rings and double-sixrings. Various combinations of SBUs create numerous framework types of different geometries, sizes and pore connectivities .
Aluminophosphates (AlPOs) are another class of microporous materials with tetrahedra of [AlO4] and [PO4] . The structure of this class of materials is similar to that of zeolites but with SBUs made of a [Al-O-P] link. The alteration between Al and P restrict the structural freedom to SBUs with an even number of T-atoms and do also introduce a larger charge distribution in the neutral framework. The introduction of silicon atoms into the neutral framework of AlPOs results in a new class of materials, called silicoaluminophosphates (SAPOs).
Three different mechanisms have been proposed for silicon substitution into the AlPO framework (Figure 1.1) . The first mechanism, SM1, consists of the substitution of an aluminium atom by silicon. The second mechanism, SM2, is a silicon substitution for phosphorous. The proton necessary to balance the charge of the framework (due to the Si substitution (+IV) for phosphorous (+V)) generates a surface bridge hydroxyl group that is known as a Brønsted acid site. The last mechanism is the simultaneous replacement of a pair of aluminium-phosphorous by two silicon atoms . The SM1 mechanism is unlikely to happen as it leads to both a positive framework charge and the formation of a [Si-O-P] link that is known to be unstable [5, 7]. The combination of the SM2 and SM3 mechanisms leads to the formation of silicon aggregates or “silicon islands” . Silicon islands are formed when at least two adjacent T sites are occupied by silicon [9, 10],.
A planar schematic of silicon incorporation mechanisms in an AlPO framework.
1.2. Silicon island formation in SAPOs One important feature of SAPO materials is the aggregation of silicon atoms to form socalled silicon islands (Figure 1.2). It is believed that silicon islands form when the silicon content reaches a threshold that is specific for each topology [12, 13]. To determine the threshold amount of isolated heteroatoms in different topologies, Barthomeuf et al.  studied the maximum content of isolated heteroatoms in different topologies of zeolites and zeotypes (SAPOs) using topological density 1. They suggested that a higher number of isolated heteroatoms can be introduced into structures with lower topological density, based on topological constraints. The topological approach was later supported by Sastre et al.  who studied the topic theoretically. While silicon island
Topological density (TD2-5) is defined as when
formation is found to be thermodynamically favoured , there are few details about how such islands form. It seems that the aggregation of silicon into islands occurs due to migration of silicon T-atoms in the presence of vacancies and is promoted by extra framework spices like water [9, 15].
The formation of silicon islands may occur during synthesis or in post-synthesis modifications, such as calcination, which occur at high temperatures. As-synthesized islands occur in the samples where the silicon concentration in the synthesis gel is relatively high. Post-synthesis island formation occurs even in samples that lack islands in the as-synthesized sample .
Planar scheme of SAPO network: (a) isolated Si, (b) 5Si island, (c) silicon island with 1 Al inside the island .
Si NMR of SAPOs gives us information about the identity of Al or Si for the atoms in the second coordination of tetrahedra around a central Si atom (Figure 1.3) and has therefore been an important tool in studying the aggregation of silicon atoms experimentally [13, 18-22]. Unfortunately it does not give precise information on size and shape of silicon islands, as many different silicon islands may be assigned to same NMR spectra.
Buchholz et al.  used a combination of chemical shifts in 29Si NMR and x-ray diffraction to study silicon island formation upon heating. They observed that as silicon atoms aggregated (according to NMR), the crystallinity of the structures remained unchanged. This indicates that silicon atoms migration and vacant sites left by silicon do not impose any crystallinity changes on the framework that are detectable by x-ray diffraction, and therefore silicon vacant sites are somehow filled to avoid crystallinity loss. According to 31P MAS NMR spectra, the migration of phosphorus atoms to silicon vacancies and their transformation from P(OAl)x(H2O)y to P(OAl)4 species seems to be the reason for the lack of changes in crystallinity during silicon island formation, a phenomenon known as structural healing .
Figure 1.3 29Si NMR spectra for SAPO-34 .
Another experimental method to study silicon islands was proposed by Martins et al.  who used a multipeak curve-fitting approach to analyse FTIR spectra in the OH stretching region of SAPO-34. Their analysis suggests the presence of three distinct acid sites (named OHA, OHB, and OHC in Figure1.4). The components OHA and OHC have been assigned to proton attached oxygen atoms of different crystallographic positions in agreement with previous studies [24-27], while the OHB sites had stronger acidity, as is expected in SAPOs. These were explained as protons at the borders of silica patches/islands, which can be used to identify the formation of silicon islands in the sample.
Figure 1.4. FTIR spectra in the region of SAPO-34 samples .