## Zeolites **Zeolites -** Open and fully-crosslinked framework structures composed of corner-sharing $\ce{SiO4}$ and $\ce{AlO4}$ tetrahedra. - **Crosslinked -** All ions are tetrahedrally coordinated with no hanging bonds. **Zeotypes -** Similar tetrahedral open framework materials that contain other elements. - **Possibilities -** Can add $\ce{P^5+, Ga^4+, Zn^{2+}}$ etc. - **Nomenclature -** The distinction between the two classes is moot in chemistry, arises from the geological background. **Important Properties -** They posses a number of important features that lead to their uses: - **Total Porosity -** Their high porosity gives them a large surface area, increasing the potential for activity. - **Pore Size -** The pores have sizes on the order of molecules, allowing activity to be shape selective. - **Crystallinity -** The material is crystalline and hence the pore size is largely invariant throughout the structure (increases selectivity). **Applications -** There are many applications of zeolites: - **Pigments** - Chromophores can be stabilised inside of the cages, *such as in Lapis Lazuli/ultramarine where $\ce{S3^{.-}}$ radicals are stablised.* - **Ion Exchange** - Very commonly used in detergents, where the $\ce{Na^+}$ cations can exchange with $\ce{Ca^2+}$ to soften it. - **Oil Refining** - Zeolites with larger pores are used for the cracking process in oil refining. - **Antimicrobials** - The pores in zeolites can be used to slowly release molecules that are antimicrobial. - **Automotive Catalysis** - Zeolites are used in diesel catalytic convertors, where they react AdBlue (urea) with $\ce{NO_x}$ molecules. - **Medical Devices** - Applications include *Quick Clot*, where the zeolites are used to absorb water and therefore concentrate the bodies natural clotting agents. - **Adsorption** - Use the energy of absorption/desorption to work like a fridge/heat pump, *e.g. self cooling keg of beer*. - **Space Exploration** - Zeolites are of similar composition to conventional soil, but can be designed to store slow-release fertiliser. Potential for space colonisation, now used for golf greens. ### Basic Structure > [!core] Chemical Formula > $\ce{M_{x/z}[(Al_{x}Si_{1-x})O2].qQ}$ > where $M^{z+}$ are the exchangeable cations, $\ce{[(Al_{x}Si_{1-x})O2]}$ is the anionic framework and $\ce{Q}$ are the sorbate molecules. > > Also can be written as $\ce{M_{m/z}[m AlO2 n SiO2].qQ}$ **Lowenstein’s Rule** - The maximum value of $x$ for $\ce{M_{x/z}[(Al_{x}Si_{1-x})O2].qQ}$ is 0.5. - **Linkages** - This is a result of Al-O-Al linkages concentrating negative charge, hence only Si-O-Si and Si-O-Al linkages are stable. **Counting -** When referring to rings etc., count the tetrahedral atoms and not the oxygens. **Building Blocks -** Zeolites are all made up of the primary building block of the tetrahedral unit, however these can be arranged into secondary units: - **Rings -** The tetrahedra will arrange themselves into rings, most commonly four and six membered rings (4R and 6R). Rings with an odd number of members are also possible, but less common. - **Double Rings -** Two rings can connect up to create double rings. These are often used to connect up proper cages. Denoted by D$n$R where $n$ is the size of the ring. - **Cages** - Larger combination of tetrahedra can create a larger variety of cages, *two common examples being cancrinite and sodalite.* - **Codes** - There is a code classification system for these cages, but this can be ignored for the course. - **Sodalite Cages** - A very common type of cage that is also called a $\beta$ cage. ![[MorrisJMatChem2005_1.gif#invert|c]] - **Ring Openings** - The size of the channels in zeolites is determined by rings formed of the tetrahedral atoms. - **Size** - The size of the rings can be classified as small-pore, medium-pore and large-pore, varying between 6R to 20R. - 6R can be a bit too small to be useful. - **Pore Network** - The pore network can have varying number of dimensions. **Topologies -** There are currently 258 zeolite structures listed on the [database](https://europe.iza-structure.org/IZA-SC/ftc_table.php). - **Sodalite, SOD** - A body-centred cubic structure of sodalite/$\beta$ cages. [^1] - **Connections** - The cages are both joined on their 6 rings and 4 rings. The 4 rings are along the \[100] direction, the 6 rings along the \[111]. ![[database1.png|cs]] - **Pore Size** - As the pores are at biggest 6 rings, the pores are barely big enough to class as zeolite. - **Uses** - Often used for pigments, such as ultramarine. - **Zeolite-A, LTA** - The sodalite cages are now joined by double 4 rings, creating a central $\alpha$ cage. [^2] - **Connections** - Can be thought of as a primitive array of $\beta$ cages connected by D4R or a primitive array of $\alpha$ cages connected by single 8 rings. ![[LTA_sod_100.jpg|cs]] - **Pore System** - Contains a 3 dimensional 8-ring channel system that runs along the \[100], \[010] and \[001] directions. - **Uses** - Used in water softeners, ion exchange, antimicrobials and nuclear clean-up. - **Properties** - Can be made easily and cheaply. - **Zeolite X/Y, FAU** - A *kinda*-face-centred cubic arrangement of sodalite cages joined by D6R.[^3] - **Comparison** - The structure forms the same arrangement as found in diamond. ![[FAU_111.jpg|cs]] - **Pore System** - The pore size is now 12 rings with a 3D channel system. - **Uses** - The larger pores make it useful for cracking etc. - **Properties** - More expensive to make (hence need high-value applications); less dense than LTA. - **EMC-2, EMT** - The hexagonal close packed equivalent of FAU, which also contains the rings of sodalite cages connected by double 6 rings. - **ZSM-5, MFI** - A zeolite structure built from 5 rings, termed a pentasil zeolite. - **Channels** - The channels create a 3D network of 10 rings, where in one direction they are straight and in one direction they are sinusoidal. - **Cages** - The cages at the intersection of the channels have interesting properties. ![[1-s2.0-S2468519422002907-gr4.jpg#invert|cl]] ^13623c - **Aluminium Content** - Due to Lowenstein’s rule, the maximum amount of aluminium is 40%. - **Uses** - Used in petrochemical industry. - **Zeolite Beta, \*BEA** - A somewhat disordered structure where there are regular 1D 12-ring channels with a disordered arrangement of T atoms inside. - **Intergrowths** - The structure is formed as 2/3 different polymorphs growing together. - **Effect -** As the channels are ordered, the disorder makes no difference to the activity. ![[Beta_random_stacking.png|cs]] **Extra-framework Cations** - In the previous structures, only the tetrahedral atoms were shown. The extra-framework cations can sit in the pores close to the walls or inside cages. - **Amount** - The amount of cations is dictated by the aluminium content, low Si materials require more cations. - **Ordering** - The ordering of the cations can vary between cations, often they can hop. **Aluminium Phosphates** - $\ce{AlPO4}$ can make a neutral framework like $\ce{Si2O4}$. - **Charge Balancing** - The charges on the framework can be introduced in two ways: - **Divalent Cations** - An $\ce{Al^{3+}}$ cation is substituted with a divalent cation, such as $\ce{Zn^{2+}, Fe^{2+}}$ or $\ce{Co^{2+}}$ - **Silicon Replacement**- An $\ce{Al^{3+}}$ cation is substituted with a $\ce{Si^4+}$ cation. - **Even Rings** - Does not work with pentasil structures, as there needs to be an equal number of $\ce{Al^3+}$ and $\ce{P^5+}$ cations. ### Synthesis **Synthesis** - In general, zeolites require a significant amount of energy to make. - **Ingredients** - The synthesis requires a source of Al/Si, a solvent and a structure directing agent. **Hydrothermal** - Therefore, they are most commonly made hydrothermally in a teflon-lined stainless steel autoclave. - **Autogenous Pressure** - The water can be heated above boiling point, creating internal pressure. - **Temperature Requirements** - The high temperature gives reversibility to the material synthesis. - **Mistake Healing** - The ability to heal mistaken bonds allows the structure to be crystalline and not a glass. - **Rate** - By heating, there is enough energy for this reversibility to occur in a realistic timeframe. **pH Requirements** - To get enough of the Al and Si in solution, a strongly basic solution is used. - **Effect on Silica** - The silica’s very slow solubility is made slightly better in basic conditions, helping material be transferred to the reaction site. - **Effect on Alumina** - In basic conditions the alumina is present as the tetrahedral $\ce{Al(OH)^-4}$ ion, whereas at lower pHs it is present as the octahedral $\ce{Al(OH2)^{3+}6}$ ion. - **HF Alternative** - Rather than using basic conditions, hydrofluoric acid can also dissolve the silica. - **Etching** - This is the same process as the etching of glass. - **Use** - Can be used in research setting, industrial applications avoided for health and safety reasons. Sometimes used for creating separation. - **Alumina** - Rather than incorporating aluminium ions, the fluoride ions can be incorporated into the structure. **Structure Directing Agents, SDA** - The addition of these species into the synthesis pot give rise to the desired porosity. - **Species** - For small pores, often inorganic cations used; for larger pores, use organic cations. These can be burned off after synthesis. - **vs Templating** - The SDAs are not true templates as the porosity does not perfectly match. - **Thermal Averaging** - While the zeolite is being synthesised, the SDA will be vibrating and rotating, making a near spherical pore. **Charge Matching** - For synthesis, there must be sufficient cations to allow the charges to balance. - **Low Si Material** - A zeolite with lots of aluminium, hence needing a lot of cations to balance the charge. - **High Si Material** - A zeolite with low amounts of aluminium, hence requiring comparatively less cations. **Zeolite Conundrum** - Despite the more than 6 million hypothetical ways to connect $\ce{SiO4}$ tetrahedra, only 200 framework structures have been found. - **Synthesis Red Line** - If a plot of density and energy is constructed, all currently-known zeolites are found along a straight line. ![[Pophale1.png#invert|cm]] %% doi.org/10.1039/c0cp02255a %% - **Origin** - By it’s very nature, the synthesis of zeolites must be reversible to get a crystalline product. However, this prevents metastable zeolites from being synthesised as the more stable product will always be formed. - **Factors** - A number of factors can characterise how easily a zeolite will form: - **Feasibility Factor** - The further away from the zeolite red line, the less feasible the zeolite synthesis is as there is greater driving force rearrangement. - **Flexibility Window** - The zeolite structure requires some flexibility during synthesis to ensure that it can form successfully. - **Local Interatomic Distance Criteria** - A set of criteria imposed on the bond angles and distances to ensure correct tetrahedral geometries are adopted. **Traditional Synthesis Vector** - The traditional synthesis process is thermodynamically-controlled; moving from a low-density gel to a high density zeolite. **ADOR Process** - A alternate synthesis technique to produce classically infeasible zeolites. - **Premise** - A weakness is engineered into the “red line” zeolite base structure than can then be used to alter the structure after the fact. - **Acronym** - ADOR stands for assembly, disassembly, organisation and reassembly. - **Weakness** - The replacement of a select amount of Si with Ge introduces a weakness to the structure. The Ge atoms naturally congregate in places and hence certain elements of the structure can be removed. - **Weak Acid** - A weak acid treatment is used to remove the Ge without damaging the silica structure. - **Organisation** - Once removed, a chemical moiety can organise the layers. - **Reassembly** - Depending on the chemical moiety, it may just burn off or could add extra silicon atoms for the structure. >[!example] Formation of IPC-4 & IPC-2 > 1. **Assembly** - The UTL base structure is formed through a conventional synthesis but with some Si substituted for Ge. > 2. **Disassembly** - The Ge is then removed from the structure via treatment with weak acid, leaving behind IPC-1, where layers are only connected via hydrogen bonding. > - **Characterisation** - These layers will now contain $\rm Q^3$-type Si atoms. > 3. **Organisation** - A chemical moiety is intercalated between the UTL-like layers, which results in organisation and a standard separation. > 4. **Reassembly** - The layers are fused together using a 540°C calcination, whereby the intercalated moiety determines the separation of the layers. > > ![[41557_2013_Article_BFnchem1662_Fig4_HTML.webp#invert|cl]] > > **IPC-2** - A silyl moiety is intercalated between the layers, resulting in the layers being linked by a S4R. > **IPC-4**- Octylamine is intercalated between the layers, resulting in purely oxygen linkers. > *See [Roth et. al., Nature Chem.](https://doi.org/10.1038/nchem.1662)* - **Benefits** - By changing the structure after the fact, metastable structures can form that would not have formed if there was reversibility. This access other areas of the plot. ### Characterisation **X-Ray Diffraction** - X-ray diffraction can characterise the atomic structure of crystalline materials such as zeolites. - **Average Positions** - Importantly, XRD only shows the average atomic positions. - **Powder** - As much as single crystal would be ideal, zeolites are often synthesised as powders and hence only pXRD can be completed. - **Structure Solution** - The use of pXRD means that a structure solution can be quite difficult to achieve and hence this must be used in conjunction with a number of other techniques. **Electron Microscopy** - TEM images can be used to view the structure directly. TEM images can highlight the presence of defects. - **Electron Diffraction** - ED is also possible in a TEM, may help in finding structure solutions. **Solid State NMR** - Also excellent for characterisation, an element-specific probe on local structure and dynamics. No long-range ordering required. - **Nuclei** - A number of nuclei in zeolites are NMR active. | Nuclei | Spin | Abundance | Comment | |:-------------:|:----:|:---------:|----------------------------------------------------------------------------------------| | $\ce{^29Si}$ | 1/2 | 4.7% | Found in all zeolites, well defined $\delta$ dependence on environment | | $\ce{^27Al}$ | 5/2 | 100% | Found in zeolites + ALPOs, quadrupolar, $\delta$ strongly dependent on CN | | $\ce{^31P}$ | 1/2 | 100% | Found in ALPOs, easy nuclei to work with | | $\ce{^17O}$ | 5/2 | 0.037% | Present in all frameworks, quadrupolar, needs isotopic enrichment | | $\ce{^1H}$ | 1/2 | 99.99% | Present in template, water or OH groups, examine Brønsted acidity | | $\ce{^23Na}$ | 3/2 | 100% | Often included for charge balancing, exchangeable | | $\ce{^2H}$ | 1 | 0.01% | Simple enrichment, study hydrogen diffusion | | $\ce{^129Xe}$ | 1/2 | 26.4% | Probe pore volume + shape, large $\alpha$ results in very environment-dependent $\delta$ | - **Experiments** - A number of experiments may be completed. - **1D Magic Angle Spinning** - A 1D experiment in any of the above nuclei; magic angle spinning reduces dipolar broadening. - **2D Experiments** - High resolution 2D experiments can probe heteronuclear and homonuclear coupling (both through bonds and space). - **Dynamics** - The broadening of peaks at different temperatures can be used to probe diffusion processes. - **Information Gained** - A wealth of information can be gained: - **Molecular Structure** - Classic organic NMR may be desirable to determine molecular structure of templates etc. - **Symmetry** - Although NMR cannot determine symmetry as XRD, the number of equivalent sites can confirm/contradict a space group. - **Coordination Number** - $\delta$ of Al is strongly dependent on its coordination number (lower CN at higher shifts). - **Network Polymerisation** - The chemical shift of Si is strongly dependent on the number of -O-Si bonds its has. (-95 ppm for $\rm Q^{3}$; - 110 ppm for $\rm Q^{4}$) ![[rem-notes-1.png#invert|cl]] - **Neighbouring Species** - In a similar way, the number of -O-Al bonds the Si centre has also affects $\delta$. Increasing -O-Al linkages changes $\delta$ from -110 to -80. - **Si/Al Ratio** - Using the intensity of these separate peaks and Lowenstein’s rule (no Al-O-Al linkages), the Si/Al ratio can be determined. $\left(\ce{\frac{Si}{Al}}\right)_{\text{NMR}}=\frac{I_{4}+I_{3}+I_{2}+I_{1}+I_{0}}{I_{4}+0.75I_{3}+0.5I_{2}+0.25I_{1}}$ - Comparison to chemical analysis can indicate extra-framework Al. - **Bond Length/Angles** - Studies have shown that both bond lengths and angles are correlated to $\delta$. **Adsorption Experiment** - The adsorption of a gas is commonly used to calculate surface area and pore size. - **Experiment -** Within a closed system, a sample is exposed to gas and the pressure drop from absorption can be recorded. This allows calculation of volume of gas absorbed, conventionally reported as the volume at 273 K and 1 bar. - **Isotherm Types -** The shape of the isotherm (all at 77 K) indicates the type of porosity present in the sample. There are five standard types. - **Pressures -** All pressures are reported in terms of $P_0$, where the gas liquefies. - **Brunauer–Emmett–Teller (BET) Analysis** - The most common type of analysis for determining surface area. $\frac{P}{V_{a}(P^{0}-P)}=\frac{1}{V_{m}C}+\frac{C-1}{V_{m}C}\left(\frac{P}{P^{0}}\right)$ - where $V_{a}$ is the volume of gas adsorbed at pressure $P$, $V_{m}$ is the volume of gas required to form a monolayer and $C$ is the BET constant. - **Rearrangement** - For determination of surface layer, $V_{m}$ is desired and hence the equation is expressed for a straight line (divide left fraction by $P$ on both sides). $\underbrace{\frac{1}{V_{a}\left(\frac{P^{0}}{P}-1\right)}}_y=\underbrace{\left(\frac{C-1}{V_{m}C}\right)}_m\cdot\underbrace{\left(\frac{P}{P^{0}}\right)}_x+ \underbrace{\frac{1}{V_{m}C}}_c$ - **Finding the Parameters** - From a plot, the following parameters can be found. $C = \frac{m}{c}+1 \qquad V_{m}=\frac{1}{m+c}$ - **Total Surface Area** - The total surface area can then be found by calculating the number of moles of gas and then using the area of a gas molecule, $A_{N_{2}}$. $SA = \frac{V_{mono}}{V_{molar}}N_{A}A_{N_{2}}$ - **Typical Range** - Typically the pressure range 0.05 to 0.30 is utilised. >[!derivation]- Deriving the formulae for $V_m$ and $C$ from BET > > ![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad-4.png|cl]] > [!example]- BET Analysis for Example Zeolite > A student has designed an experiment to make a new zeolite. The X-ray diffraction pattern shows a highly crystalline material. Adsorption of nitrogen gas at 77 K gives the following data. Calculate the BET Apparent surface area. (Note that the mass of the sample in the experiment is exactly 1g and the surface area required by a nitrogen molecule is $1.62\times10^{-19}\rm\ m^2$. One mole of nitrogen gas occupies a volume of $22.4\rm\ dm^{3}$) > > ![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad-5.png]] **Common Surface Areas** - Zeolites have surface areas of 300-800 m²g⁻¹; normal MOFs have surface areas between 1000-7000 m²g⁻¹; can go past 10 000 m²g⁻¹. ### Applications **Ion Exchange** - The extra-framework cations present in the structure can be exchanged for an alternate ion to remove it from the environment. - **Water Softening** - Exchange of $\ce{Na+}$ with $\ce{Ca^2+}$ from the environment allows the softening of water, increasing the efficacy of soaps/reducing limescale. - **Nuclear Waste Clean-up** - Radioactive $\ce{^90Sr^2+}$ can be removed from the environment by the zeolites either via scattering on the ground or filtering contaminated water. - **Cation Exchange Capacity, CEC** - The amount of cations that can be exchanged is entirely dependent on the amount of aluminium in the zeolite. >[!example] Calculating Cation Exchange Capacity >The cation exchange capacity can be calculated under the following assumptions: >- All sodium ions are exchangeable. >- The water content is unaffected by cation exchange. > >![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad-2.png]] **Catalysis** - A significant use case of zeolites is for catalysis, where they succeed for a diverse range of reactions. - **Separation** - Zeolites are superior to homogenous catalysis as they allow very easy separation after the reaction. - **Crystallinity** - The high crystallinity of the zeolite leads to high specificity as the pore’s have very regular sizes. - **Surface Area** - The high surface area of zeolites results in a large number of possible reaction sites. - **Tunability** - The structure and catalytic sites are highly tuneable via composition changes, addition of transition metals, surface modifications etc. - **Safety** - Zeolites are non-toxic and non-corrosive. **Mechanisms of Selectivity** - There are 3 ways that zeolites can be selective. 1. **Reactant Selectivity** - The pore network will limit the size of the reactants that can reach the reaction site. 2. **Product Selectivity** - If the product exists in an equilibrium with multiple others, if only one product can escape then due to Le Chatelier’s principle the reaction becomes product selective. ^226535 3. **Transition State or Spatio-Selectivity** - The pore size may constrain the possible transition states that the reaction can adopt, therefore resulting in product selectivity. ![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad.png|cl]] **Catalytic Functionality** - Predominately, zeolites are used as an acid catalyst, although basic catalysis is also possible. - **Acid Catalysis** - If the extra-framework $\ce{Na+}$ is exchanged for $\ce{H^+}$, these protons make for excellent Brønsted acid catalysts. - **Lewis Acidity** - As well as the Brønsted acidity, Lewis acidity can be achieved via addition of extra-framework $\ce{Al^3+}$ ions, which can be created via thermal treatment. - **Base Catalysis** - Alternatively, basic catalysis can be achieved via the framework oxygen sites or guest species such as metal cations or metal oxides. ![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad-1.png|cl]] **Adjusting Acidity** - The acid site is very tuneable depending on the structure around it. - **Aluminium Content** - Every proton requires an aluminium cation for charge balancing and therefore the number of acid sites depends on the Al content. - **Strain** - The level of strain on the bond has a significant effect on the acidity. - **Mechanism** - More strained structures result in changes to orbital overlap for the OH and hence ease abstraction and increase acidity. - **Characterising Strain** - The strain can be measured by $\omega$, the angle between the tetrahedra, and $b$ the distance between the two closest tetrahedral atoms. - **Linear Relationship** - Structure-property relationships have been found that link the $b$ and $\omega$ to the acidity or O-H bond wavenumber. - **Relieving Strain** - Addition of tetrahedral ions larger than $\ce{Si^4+}$ will relieve some of the strain and make for weaker acidity. - **Trade-off** - As $\ce{Al^3+}$ is a bigger ion, there is trade-off between acid strength and number of sites. *e.g. Zeolite A quite weak due to high number of $\ce{Al^3+}$. - **ADOR Process** - The ADOR process allows access to highly strained structures due the irreversibility of the process. - **Measuring Strain** - A way of measuring the strain in ADOR created compounds is measuring the area of a unit. Strained structure will buckle and therefore have a lower area. **Redox Functionality** - Additionally, redox functionality may be added to the zeolite. - **Isomorphous Substitution** - Redox active ions can be substituted into the framework structure such as Ti, V, Cr or Co. - **Immobilised Complexes/’Ship-in-a-bottle’** - Normal homogenous complexes can be synthesised inside the pores in a manner where they are trapped. - **Separation** - This makes for significantly easier separation at the end of the reaction, as long as the reactants can make it to the pores. - **Pt Clusters** - A similar thing can be achieved with Pt nanocluster and aggregates. **Organic Reactivity** - Zeolites can rather a lot of organic chemistry, especially as many organic reactions are acid catalysed. - **Examples** - Includes nucleophilic substitution, electrophilic addition, aromatic substitution, eliminations, cyclisation etc. >[!example] Zeolites in the Oil and Petrochemical Industries >**Refining** - The transformation of complex hydrocarbon feedstocks (*i.e.* crude oil) into a broad range of similar products. > >![[rem-notes-2.png#invert]] > >- **Basic Idea** - Distil the oil and then treat with some chemical process to generate more of desirable products. >- **Complexity** - Oil refining is a complex process with many steps, many of which include zeolites as catalysts. >- **Fluid Catalytic Cracking, FCC** - One of the most important process that accounts for over 90% of the annual consumption of synthetic zeolite catalysts. > - **Use** - Converts less desirable C20-C30 hydrocarbons and cracks them into more desirable C7-C10 hydrocarbons (good for gasoline). > - **Catalysts** - Predominately uses zeolite Y (FAU), some ZSM-5 (MFI) also used. > - **Process** - An acid-catalysed carbenium ion rearrangement. > - **Selectivity** - The desirable gasoline hydrocarbons diffuse out the zeolite fast enough to prevent further cracking and coke formation (pure C). > - **Advantages of Zeolites** - As well as the selectivity, much safer than an equivalent acid catalysis ($\ce{H2SO4}$). > >**Petrochemistry** - The transformation of well defined feedstocks (the products of refining) into well defined products. >- **Zeolites** - Zeolites are also widespread for petrochemistry, such as for dealkylation or methanol to olefin reactions. >- **[[#^13623c|ZSM-5 (MFI)]]** - Widespread for use in these applications. > - **Advantages** - Highly porous structure, very acidic Al sites. > - ***e.g.* Aromatic Isomerisation** - An example reaction with ZSM-5 is the conversion of toluene to a more valuable mixture of *p*-xylene and benzene.![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad-3.png#invertW|cl]] > - **[[#^226535|Product Selectivity]]** - The reaction is selective towards to *p*-xylene as it has a diffusion coefficient 10 000 times the other isomers. > - **Avoids Side Reactions** - The pore size prevents the bimolecular disproportionation of xylene to toluene and trimethylbenzene. > - **Usefulness** - The ease of this reaction is a key reason for the ubiquity of polystyrene which uses a monomer derived from *p*-xylene. >- **TS-1 (MFI)** - An alteration of the ZSM-5 structure to substitute $\ce{Ti^4+}$ cations into the framework. In conjunction with hydrogen peroxide, makes for a great organic oxidising agent. > - **Convivence** - Much cheaper and more convenient than some organic peroxide molecule such as *m*-CPBA and produces water by-product. ## Metal Organic Frameworks **Coordination Polymer** - A coordination compounds continuously extending in one, two or three dimensions through coordination bonds. **Metal Organic Frameworks, MOF** - A coordination polymer with an open framework structure that contains potential voids. - **Potential Voids** - The structure may come apart when the solvent etc. is removed. - **Composition** - A MOF is composed from two things - a metal centre (not necessarily tetrahedral) and organic linkers. - **Ultrahigh Surface Areas** - MOFs achieve incredibly high surfaces areas for a crystalline material, up to 10 000 m²g⁻¹. ### Structures **Inorganic Cluster** - The metal part of the MOF. Can be just one metal ion or can be multiple put together in a cluster. - **Coordination Geometry** - The tetrahedral constraint on the geometry found in zeolites is no longer present. **Organic Linkers** - A variety of organic compounds can be used as organic linkers between these clusters. - **Requirements** - Need multiple places where metal ions can bind. - **Common Examples** - Tend to either be carboxylate groups or *N*-heterocycles. *Terephthalic acid is a classic example.* ![[mechemexpress.gif#invert|cs]] **Secondary Building Unit, SBU** - A simplified substructure motif which is used to build the overall structure. - **Nodes** - These SBUs can act as nodes in the network. - **Simplification** - The SBU of an inorganic cluster only shows the points where it links with the organic linkers. Similarly, the SBU of an organic linker is only contains where it connects to metal clusters. ![[yaghi-paper-1.webp#invert|cl]] %% https://doi.org/10.1038/nature01650 %% - ***e.g.* Paddlewheel Units** - Structure (b) is a paddlewheel unit, which contains two metal centres in the blue tetrahedra. - **Connection to Linkers** - It only connects to the organic linkers in one plane at four points and hence its SBU is a square. **Structural Diversity** - With so many choices for inorganic clusters and organic linkers, there is almost an unlimited number of MOFs. **Zinc Imidazolate Frameworks, ZIFs** - Adopt structures that are effectively enlarged zeolite structures due to the similar bond angle. - **Bigger Pores** - The larger size of the imidazolate linker results in larger pores. - **Uniqueness** - Not many other MOFs adopt structure like these. ![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/wikipedia-1.png#invert|cl]] **MOF-5** - The first super-porous MOF, composed of terephthalic acid linkers and four metal clusters ($\ce{M4O(O2CR)6}$). SBU is an octahedra. - **Instability** - MOF-5 is unstable in water and hence not widely used. - **Surface Area** - It has a surface area of *ca.* 3 500 m² g⁻¹. **HKUST-1** - Combination of a paddlewheel 2 Cu cluster and benzene-1,3,5-tricarboxylic acid. ![[wikipedia-2.png|cs]] - **Open Metal Sites** - The SBU of the paddlewheel cluster is a square, but there are two additional sites on each cluster for coordination. - **Occupancy** - After synthesis occupied by water, but can be replaced or driven off. - **Cu-Cu Bond** - Some diagrams may indicate a Cu-Cu bond in the cluster but this is not real as the $d_{z^{2}}$ orbitals are fully occupied. **CPO-27 or MOF-74** - Many possible metals linked with 2,5-dihydroxy-1,4-benzenedicarboxylic acid. - **Pore Structure** - Pores create 1D channels through the structure with chains of metal running parallel. - **Open Metal Sites** - Has a very high density of open metal sites that allow the attachment of many guest species ![[wikipedia-3.png|cm]] %% https://commons.wikimedia.org/wiki/File:CPO-27_OrthographicView_CrossSection.png %% **Flexible MOFs** - MIL-53 has a topology that allows compression along one axis, just like a wine rack. - **H₂O** - The addition of water results in the pores being reversibly squeezed shut due to the formation of hydrogen bonds. - The flexibility is an advantage of MOFs, there are no corresponding flexible zeolites. ![[fchem-05-00111-g001.jpg#invert|cl]] ### Synthesis **Synthesis** - The different type of bonding in MOFs requires a different type of synthesis than zeolites. - **Zeolite Comparison** - Zeolites contain similar strong covalent bonds, whereas MOFs have both strong covalent bonds within the linker but also weaker coordination bonds. - **Low Temperature** - The weaker coordination bonds can be reversible at lower temperatures and therefore the synthesis needs less energy Also less issue with pH. - **Linker** - We are not aiming is alter the linker in the synthesis process. - **Crystallisation Process** - Can be completed in a number of ways. - **Slow Evaporation** - Mix together metal and linker solutions and slowly evaporate the solvent. - **Layering Solutions** - If two immiscible solutions are layered, can quite often grow nice large crystals at the interface. - **Slow Diffusion** - May want to slow down reaction for better crystallinity by slowly diffusing a solution through a membrane or immobilizing gel. - **Solvothermal Techniques** - May still be required or allow faster synthesis. May also give more robust porous structures. **Reticular Synthesis** - A series of synthesises that utilise the same structural connectivity but a different organic linker or metal centre. - **”Reticular”** - Characterised by a fine network or netlike structure. - **Isoreticular Series** - A series of MOFs that share the same connectivity but have varying organic linkers, likely resulting in variation in size. ![[soreticular-series-of-MOF-5-with-dicarboxylate-organic-linkers-varying-length-70-Reused 1.png#invert|cm]] **Interpenetration** - The mutual intergrowth of two or more networks in a structure where the networks are physically intertwined but not chemically linked. - **Space Filling** This tends to occur as the pores get bigger, the structures want to interpenetrate to fill the void space. - **Use** - As the porosity is pretty key to the usefulness, this is generally undesirable. **Post-Synthetic Modification, PSM** - It is possible to make modifications to the covalent bond framework in the MOF after the structure is created. - **Functionalisation** - A wide range of functional groups can be added without any constraints placed by the MOF synthesis conditions. - **Isolation and Purification** - The isolation of the modified MOF is facile compared to the homogenous modification of a linker before the MOF synthesis. - **Simplicity** - A wide range of topologically identical but functionally diverse MOFs can be very simple to access. - **Multiple Modifications** - Two modifications can be done on the same MOF, allowing a mix and match of properties. ### Applications **Applications** - Many different applications for MOFs. - **Gas Storage** - High surface are enables storage of gases such as fuels or medical gases (*e.g.* NO, CO). Better for storage of dangerous gases as less volatile. - **Gas Capture** - Potential use for carbon capture. - **Gas Separation** - Can separate olefins from alkanes. - **Drug Delivery** - Can potentially serve as a vessel for drug delivery. - **Others** - Other applications include catalysis and sensors. **Gas Storage** - MOFs are significantly better than zeolites for gas storage due to their much higher surface area. - **Physisorption** - The adsorption of gas molecules on a surface. The bonding is enough to keep them there but allows them to be easily removed. > [!example] Hydrogen Storage > - **Need for Hydrogen Storage** - Hydrogen fuel cells are a potential solution for the electrification of electric vehicles. Success of these vehicles requires a large amount of $\ce{H2}$ to be stored to give sufficient range. >- **Hydrogen Economy** - This is envisaged to integrate into the hydrogen economy, where hydrogen is electrolysed from water via solar energy. >- **Storage Requirements** - We want to be able to rival the storage of a fuel tank, which has remarkably good energy densities. Current technologies have issues: > - **Compressed $\ce{H2}$ Gas** - High pressures of over 700 bar can successfully store the hydrogen, but is a significant explosion hazard. > - **Cryogenic Storage** - Liquid $\ce{H2}$ is orders of magnitude more dense, but it is energy consuming to condense it and keep it cold. > - **Chemisorption Methods** - Chemical storage of hydrogen in metal hydrides does work, but some irreversibility, poor kinetics and have to carry around the useless metal ions. >- **MOFs** - High surface area MOFs are promising for this application due to their very high surface area, tunability and ability to be functionalised. > - **MOF-5** - Original tests on MOF-5. At 77 K has promising adsorption properties, but this is not recreated at room temperature. >- **Heat of Adsorption** - Currently, the issue is that the heat of adsorption is too low at *ca.* 5 kJ mol⁻¹. This means thermal energy can easily break the adsorption. > - **Target** - Increasing this to 15 - 30 kJ mol⁻¹ while maintaining the same surface area would fix this problem. >- **Current Research** - Current research has come to some conclusions: > - **Surface Area** - The uptake of $\ce{H2}$ is approximately linear with the surface area of the MOF. > - **Functionalisation** - It appears that the linker has little effect on $\ce{H2}$ uptake. > - **Corners** - Interaction energy is increased at corners (two surfaces to bind to) but this is in direct competition with high surface areas. > - **Open Metal Sites** - Open metal sites also have greater adsorption energies, but it difficult to get enough to make a large difference. >- **Conclusion** - Improvements to MOF $\ce{H2}$ storage has largely stalled. >[!example] Storage of Other Gases >- **Methane** - Storage of methane is much more promising as it naturally has higher heats of adsorption > - **Benefits** - Although not quite as environmentally friendly, methane fuel cells still offer better efficiencies to combustion engines and less pollutants. > - **Current State** - It is possible to store as much methane via adsorption than compressed natural gas used currently. >- **Toxic Gases** - Storage of toxic gases such as arsine is another potential application. > - **Benefits** - Care less about densities, more about the safety of having less compressed toxic gases. > - **Safety** - Store at 0.9 atm, meaning a leak will result in the the MOF absorbing from the environment rather than leaking. > - **Usage** - Widely used in the semiconductor industry, where it is pumped out to desorb. #### Case Studies >[!example] Absorbing Water > **Idea** - Absorb water from the environment using MOFs while using no power. > - **Night** - At night, the relative humidity increases (as the air cools) and hence the MOF should adsorb water. > - **Day** - The MOF is placed in a ‘mini-greenhouse’ and is heated up, resulting in the MOF releasing the water. > - **Alternatives** - Zeolites and silica gel have also been investigated, however they either uptake too little water or require too much energy to release it. > > **Swing Adsorption** - For this to work, the MOF has to exhibit pressure or temperature swing adsorption, where a small change in temperature or (partial) pressure will result in a large change in the amount adsorbed. > > ![[St Andrews/02 Public Notes/02.02 University Notes/CH5518 Blockbuster Solids/attachments/ipad-6.png|cl]] > > **Prototype MOF** - MOF-801 was selected as the initial candidate for this application. > - **MOF-801** - Composed of $\ce{Zr6O4}$ clusters linked by fumarate ions. Contains three different sizes of pores. > - **Material Choice** - Both components are widely available and non-toxic. > - **Choice** - It does not have the biggest surface area (can’t adsorb loads) but does have the required adsorption isotherm. > > ![[rem-notes-3.png#invert|cm]] > >- **Selection** - It can be seen that although MOF-801 has the least capacity, it has the most accessible pressure swing for desert humidities. > - **Working Capacity** - What is more important is how large the accessible capacity is under the reachable conditions. > > **Refinement** - The original MOF-801 has been refined to one with Al metal centres over Zr for economic reasons. Additionally a solar-powered heater has been added to further improve the temperature swing. > > [Kim. et. al., *Science*](https://doi.org/10.1126/science.aam8743) >[!example] Carbon Capture >**Carbon Capture** - The capture of $\ce{CO2}$ is obviously very desirable for fighting climate change but it is quite a difficult task >**Requirements** - Must be durable, scalable and not too expensive. >- **Alternatives** - Amine and solvent systems have also been explored for carbon capture, however regeneration is energy intensive and can lead to decomposition. >- **Flue Gas** - The flue gas that is being treated is 80+% $\ce{N2}$ and $\ce{CO2}$, 7% $\ce{H2O}$, 7% $\ce{O2}$ and a variety of gases such as $\ce{CO}$ and $\ce{NO_x}$. The gas may be somewhat acidic. > >**MOFs** - All MOFs can adsorb $\ce{CO2}$ but we want a MOF that adsorbs $\ce{CO2}$ selectively. >- **Candidate Material** - The uninspiring CALF-20 MOF was selected, which contains Zn clusters that bind with triazole and oxalic acid. > >![[internet-1.png#invert|cm]] > >- **Properties** - Both the surface area and $\ce{CO2}$ adsorption compare unfavourably to MOF-5 due to its relatively small pores. > >**Selectivity** - The key property to look for is selectivity, it should adsorb $\ce{CO2}$ over $\ce{N2}$ and $\ce{H2O}$. >- **Over $\ce{N2}$** - It is not too difficult to get selectivity over nitrogen as $\ce{CO2}$ has stronger intermolecular interactions. >- **Over $\ce{H2O}$** - Much more challenging to do. CALF-20 is relatively successful it has weak interactions with water and adsorbs $\ce{CO2}$ preferentially below 40% humidity. > - No good binding sites to water, binding to $\ce{CO2}$ completed via predominately dispersion interactions. > - Water instead is absorbed in a capillary condensation like manner and hence the presence of $\ce{CO2}$ prevents this. > >![[rem-notes-4.png#invert|cm]] > >**Evaluation** - A MOF can be evaluated for carbon capture applications via breakthrough experiments, where gases are pushed through a MOF filter. >- **Breakthrough Time** - The time it takes for the particular gas to get past the filter corresponds to when the MOF has adsorbed as much as it can. > >**Regeneration** - After the MOF is filled, it can be regenerated easily so it can be used again. > >**Testing** - CALF-20 has undergone large scale tests where it performed admirably, it was durable with respect to water (checked the structure doesn’t decompose) and was scalable. > >[Lin et. al., *Science*](https://doi.org/10.1126/science.abi7281) [^1]: An example is $\ce{|Na+8Cl^-2|[Al6Si6O24]}$, which is a cubic $P\bar 4 3n$ structure with a = 8.870 Å. [^2]: An example is $\ce{|Na+12|[Al12Si12O48].27H2O}$, which is a cubic $Fm\bar3c$ structure with a = 24.61 Å. [^3]: Not quite fcc, two intersecting fcc lattices whatever that means. An example is $\ce{|(Ca^{2+}, Mg^{2+}, Na^{+}_{2})_29|[Al58Si134O384].240H2O}$, which is a cubic $Fd\bar3m$ structure with a = 24.74 Å. [^4]: