Advanced Functional Porous Materials Exam 1

Relevant research areas for porous materials

-Zeolites and MOFs as membranes
-Molecular Gas Separations
-Natural Gas Storage
-Heterogeneous Catalysis
-CO2 conversion to chemicals
-Synthesis of biofuels

Goal of this course (1)

Develop basic chemical strategies for making nanomaterials which can serve as tailored functional materials

Goal of this course (2)

Integrate fundamentals and functional applications of porous materials from micro to macroporous regime

Nanochemistry Definition

The utilization of synthetic chemistry to make nanoscale building blocks of different:
-size
-shape
-compsition
-surface structure
-charge
-functionality

Heterogeneous catalysis

Tailor properties, concentration of active sites, to impact product and yield

We want A->B, but products C and D also form... how to we optimize the reaction pathway to get the product we want?

Microporous size regime

What are microporous materials good for?

Gas separation; pores are similar in size to CO2, CO, etc.

Mesoporous size regime

2-50nm

What are mesoporous materials good for?

Catalysis, gas STORAGE

Macroporous size regime

>50 nm

What are macroporous materials good

Catalysis, gas adsorption

What type of pores do zeolites and MOFs have?

microporous

5 principles of nanochemistry

1. Existence of Building Blocks
2. Attractive and Repulsive Interactions between building blocks
3. Association and dissociation of building blocks
4. Building block interactions with solvents, interfaces, and templates
5. Building block dynamics (ass transport)

1. Existence of Building Blocks

These are the chemical components needed to start developing nanoporous materials

2. Attractive and Repulsive Interactions between building blocks

These are needed to have long range porocity and assemble the material from building blocks.

3. Association and dissociation of building blocks

The most stable material forms from the lowest energy structure

4. Building block interactions with solvents, interfaces, and templates

For synthesis

5. Building block dynamics (ass transport)

Mass transport mechanisms that allow building blocks to assemble

Isotherms classification

Materials are characterized in terms of their pore sizes, derived from gas sorption data

What are on the axis of the isotherms?

x-axis: relative pressure p0/p
y-axis: amount of gas adsorbed

Which isotherms are microporous?

Type 1

Which isotherms are nonporous or macroporous?

Type 2, 3, 6

Which isotherms are mesoporous?

Type 4,5

Type 1

Type 2

Type 3

Type 4

Type 5

Type 6

Which isotherms are associated with stronger fluid-solid interactions?

Types 2 and 4
Useful for storing gases

Which isotherms are associated with weaker fluid-solid interactions?

Types 3 and 5
Not great for reacting gases inside the pores

What are the hysteresis loops of types 4 and 5 associated with?

Capillary condensation in the mesopores, openings that are wide from the outside but small from the inside

Two types of microporous materials

Zeolites
MOFs

What is a zeolite?

An inorganic crystalline structure with unimodal micropores

What type of bonds characterize zeolites?

covalent bonds

What applications are zeolites good for?

1. SELECTIVITY IN REACTIONS: only certain things can fit inside
2. CATALYSIS: common in petrochemical industry
3. MOLECULAR SIEVES: separate gases
4. DETERGENTS: Hydrophobic and hydrophilic functionality

What makes zeolites appealing?

High thermal, mechanical, and chemical stability

What does high chemical, thermal, and mechanical stability mean for zeolites?

It allows us to play with shape, size, and interactions of the building blocks to get the same material chemically, but with tailored properties that we select. It also helps them survive in the harsh conditions of industry.

What is one parameter we can manipulate to our advantage?

Surface area... increasing surface area makes it more useful for separation

What does MOF stand for?

Metal organic framework

What is a MOF?

An open tetrahedral structure

What are five attributes of MOFs?

1. HIGH SURFACE AREA: 1000-2000 m^2/g
2. THERMALLY STABLE
3. CHEMICALLY STABLE
4. MICROPOROUS CRYSTALLINE STRUCTURES: Functionality
5. POLAR WALLS: Uncoordinated nitrogen atoms

Why are MOFs considered hybrid materials?

They have an inorganic part coordinated to an organic molecule

What is a difficulty in MOF synthesis?

Finding the optimal conditions to form these things thermodynamically

Which is more stable... MOFs or zeolites?

Zeolites, MOFs degrade around 400 C

What is so attractive about the polar walls?

They contain a free uncoordinated nitrate that can coordinate with other molecules upon separations

What does ZIF stand for and what are they?

Zeolitic Imidazolate Framework: A mix between a zeolite and a MOF

ZIF composition

Transition metal atoms (Zn, Co) replace T atoms (tetrahedral atoms: Si, Al, P) and imidazolates replace bridging oxides in zeolites

Mesoporous materials (types, attributes, pore size)

High SA materials
Ceramics, Polymers, Composites
Unimodal pores in the 2-50nm range

Liquid crystal synthesis

Put surfactant micelles in solution so that they form rods and then hexagonal arrays. Silica will form around these arrays and, when the surfactants are calcined out, the porous material is left over: mixed metal oxide

What do micelles help with in terms of tuning?

We can choose different tails, etc. for them to play with pore sizes. Can also change wall composition

Silica (composition, uses)

SiO2
Was previously amorphous, now can be made mesoporous
Useful for catalysis

Macroporous materials

Start with colloidal array of spheres, inorganic precursors fill in spaces between them (condensation), remove the template, yields a macroporous oxide

How to form macroporous materials

High SA materials
Ceramics, polymers, composites
with unimodal pores >50 nm

6 properties of ordered porous materials

1. UNIMODAL PORE SIZES
2. High SA
3. THERMAL AND CHEMICAL STABILITY
4. CRYSTALLINE OR AMORPHOUS STRUCTURES (Functionality)
5. SURFACE CHEMISTRY MODIFIED
6. ENHANCED MASS TRANSFER (catalysis, separations, adsorption)

1. UNIMODAL PORE SIZES

molecular sieving properties, improved supports

2. High SA

50-5000 m^2/g

3. THERMAL AND CHEMICAL STABILITY

Wall nature

4. CRYSTALLINE OR AMORPHOUS STRUCTURES

Functionality

5. SURFACE CHEMISTRY MODIFIED

We can tune things to what we want

6. ENHANCED MASS TRANSFER

catalysis, separations, adsorption

Crystalline definition

Having both short AND long range order

Amorphous definition

Having short range order only, maybe more chemically reactive (?)

Which is better for applications, crystalline or amorphous?

Crystalline

What equation governs XRD?

Bragg's law

What is the equation for Bragg's law? What does everything stand for?

n(lambda)=2dsin(theta)
n=integer
lambda=radiation wavelength
d=interplanar spacing/space between atoms
theta=angle of incidence

How does XRD work?

Xray hits the lattice and scatter at specific angle, peaks of specific intensity are observed and are associated with a particular crystalline structure

Amorphous XRD peak vs crystalline XRD peak

Amorphous: BROAD
Crystalline: Narrow

What are the Baravais lattices?

There are 7 total lattice structures that materials form in (ex: cubic, tetragonal, hexagonal)

What is coordination number?

How many other atoms one coordinates to in a lattice structure

Polymorphic forms of TiO2 (3)

Rutile
Anatase
Brookite

What causes a different polymorph to form?

Different temperature and pressure conditions

Which is the best polymorph for catalysis?

Anatase: most active due to its degree of packing and higher concentration of active sites

What are the axis of XRD?

x-axis: 2theta
y-axis: intensity

How does XRD help us distinguish different facets?

Each material has a specific theta and intensity, different positions of theta can tell us which facet we have

(RED!) What important info can we get from XRD?

1. Determination of Crystal structure
2. Evaluation of crystal size
3. Evaluation of crystal orientation
4. Identification of crystalline phases
5. Evaluation of strain

2. Evaluation of crystal size

Scherrer eqn, smallest crystalline size precedent in the sample

3. Evaluation of crystal orientation

(101) found in highest peak

4. Crystalline phase identification

tetragonal, orthorhombic, could be many at once... can get fraction of each

5. Evaluation of strain

position of the peaks, what strain of material do we have?

What can we calculate from the scherrer eqn?

Crystal size

Write out debye scherrer eqn and what do each of them mean?

Gamma=[(k)(lambda)]/[(Beta)(costheta)]

Gamma: crystal size
k: Shape factor
Beta: Full width at half max
lambda: x-ray wavelength
theta: Bragg angle

SAXS

Small angle Xray Scattering

What info can we get from SAXS?

1. Evaluation of periodicity and symmetry at the nanoscale
2. Evaluation of shape, size, and separation of nanoscale objects in an ensemble

2. Evaluation of shape, size, and separation of nanoscale objects in an ensemble

Size has to do with periodicity, well defined peak at low theta means mesoporous ordered material

Do you need a crystalline sample for SAXS?

No!

What info can we get from Gas Adsorption

1. BET surface area
2. Pore size
3. Pore volume
4. Isotherms (Micro, meso, macroporous nature)

What is BET theory?

A rule for the physical adsorption of gas molecules in surfaces. It is the basis for an important analysis technique for the measurement of the specific surface area of a material

Graphs of Mesoporous gallium oxide with narrow vs broad size distribution

Know the difference: Hysteresis loops shifted farther right (I think?) for broader distribution

Xenon vs Krypton Graph

Xe adsorbs 3x more than Kr at a particular pressure

FTIR

Fourier Transform Infrared Spectroscopy

What info does FTIR tell us?

1. Assignment of lattice vibrations
2. Identification of functional groups
3. Identification of whole molecules

How does FTIR work?

A beam of IR light is passed thru a sample and examination of the transmitted light reveals how much energy was absorbed at each wavelength

What spectrum can be made?

Transmittance or absorbance, and these show the wavelengths where absorbance happens.

Sapo-34 FTIR

Sapo shows asymmetric TiO4 tetrahedra stretch, T-O bending, O-H bending

Anatase FTIR

Shows O-H only

SEM

scanning electron microscopy

What can we learn from SEM?

1. Surface morphology
2. Resolution of 1nm or larger features
3. Chemical analysis
4. Distribution of elements in the sample

3. Chemical analysis

Tells us that it is 20% X element, 40% carbon, etc.

How does an SEM microscope work?

Images sample surface by scanning it with a high energy beam of electrons. Electrons interfere with atoms in sample to produce signals that contain info about surface TOPOGRAPHY and COMPOSITION