Spectroscopy Presention

04/18/2023 1

Application of In-situ FTIR Spectroscopy in Catalysis

Koushik Ponnuru

04/18/2023 2

Importance of In-situ Spectroscopy : Comparison with

conventional offline analysis

Theory of FTIR

Instrumentation and sampling : ATR FTIR Spectroscopy

How to interpret Infra-Red spectra

Case study – Convergent synthesis of alkenes by cross

coupling of carbenes

Outline

3

In Situ FTIR Spectroscopy: MotivationMonitor reactions In Situ under Actual Reaction Conditions

Effectively monitor reactions that have components sensitive to oxygen water vapor or temperature.

Eliminate or reduce the need for offline analysisComprehensive concentration information in real-time

Follow instantaneous changes in reactants, intermediates and products providing a molecular video of reaction chemistry

Eliminate time delay associated with offline analytical methods

Study reaction kinetics , Elucidate Mechanism and Pathway Real-time reaction kinetics information Monitor changes in functional groups provides in-depth

understanding of reaction mechanism and pathway

In-situ Spectroscopy : Data Acquisition

ATR raw data

0 100 200 300 400

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Co

nc

en

tra

tio

n (

mo

l/L)

Time (min)

concentration of phenol concentration of cyclohexanol concentration of cyclohexanone

180

200

220

240

260

280P

res

su

re (

ps

i)

UV-Vis probe

Raman probe

ATR-IR 300 C, 35 bar max.

4

OH

O

OH

+ 2H2

+ H2

+ 3H2+ H2- H2O

ATR analyzed data

Kinetic Analysis: Off-line technique

Diels Alder Reaction Large number of separate

experiments required Concentrations are distorted from

synthetically relevant conditions

Drawbacks

Kinetic Analysis : In-situ Technique

[1]0 [2]0 [e] = [2]0 – [1]0

= [2] – [1]Exp. A 0.2M 0.6 M 0.4 MExp. B 0.2M 0.4M 0.2M

JACS, 2007,129,15100.

Diels Alder Reaction

Kinetic Analysis : In-situ TechniqueRearranged rate equation Rate equation

Two experiments via in-situ technique

Two SETS of experiments via classical offline analysis technique

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• Chemical bonds between the atoms within the molecules vibrate. • A diatomic molecule is considered as a two spherical masses (m1

& m2) connected with a spring with a given force constant k. • Applied Freq. = Natural Freq. of Vibration • Absorption of IR radiation takes place and a peak is observed.

Working Principle of IR Spectroscopy

1ν

2π μ

k

c

Working Principle of IR Spectroscopy

The Harmonic Oscillator

Can Model the fundamental Vibrations between the atoms using Hook’s Law.

The potential energy curve of such an oscillator is parabolic in shape and symmetrical about the equilibrium bond length.

V

+A

Displacement

-A 0

-A

+A

0

Δn = ±1

only certain energy levels exist :

1 ;

2

1 ;

2

1

2

nE n hv E h

k vv v

c

kv

c

Transition moment Integral 0 ; if =const

( ') and ( ) are orthogonal

M

21.

2V k x

Morse Potential Model Bond dissociate if stretched

beyond a certain distance

Bond length cannot be less

than minimum inter nuclear

distance

In the case of the Anharmonic

Oscillator, the vibrational

transitions no longer only obey

the selection rule n = 1.

Vibrational transitions with n

= 2, 3, ... are also possible A more accurate description of the vibrational energies is given by the anharmonic oscillator (also called Morse potential) with energy of

Modes of vibrations

R H bending

R

RHH

R

RHH

R

RHH

R

RHH

R

RHH

R H

antisymmetric

symmetric

rockingscissoring

in-planebending

stretching

Region Origin of the absorption

NIR Overtones and combination bands of fundamental molecular vibrations

MIR fundamental molecular vibrations

FIR molecular rotations

1

2

kv

c

Most organic functional groups exhibit MIR absorption spectrum High Molar absoptivities Ideal for Qualitative and Quantitative analysis

04/18/2023 12

Working principle of Interferometer

0,x n

( 1/ 2)x n

Moving mirror

Zero path diff

Moving mirror

Fixed mirror

IR light source

Photodetector

Beam Splitter

Constructive interference

Destructive Interference

Power signal

04/18/2023 13

Fourier Transform of an Interferogram

Optical Path Difference Wavenumbers cm-1

Rela

tive

Inte

nsity

Optical Path Difference Optical Path Difference

Sign

al fr

om

phot

o de

tect

or

Sign

al fr

om

phot

o de

tect

or As we move the mirror the optical path difference changes

By taking the fourier transform we can identity the frequencies present in the interferogram

Interferogram IR Spectrum

14

Measuring a typical IR Spectra

% T = 100 x I/I0

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Absorbance / TransmittanceLambert Beer Law

A = (absorptivity coefficient) x b (path length) x c (concentration)

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ATR Sampling : Method of Measurement

ReactIR 45m FTIR Spectrometer

ATR Probe

Experimental Apparatus

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• The infrared radiation interacts with the sample through series of standing waves called evanescent waves.

• An evanescent wave is produced each time the infrared beam is reflected from the inside surface of the crystal

• The evanescent waves penetrate the sample at each reflection point.

ATR Sampling : Method of Measurement

At the reflection point

2-3 µm

04/18/2023 18

Multiple Reflection

Enhanced sensitivity makes it Ideal for analyzing samples when the component of interest is low.

70% Iso propyl alcohol

04/18/2023 19

How to Read / Interpret an IR Spectra ??

Fingerprint RegionFunctional Group Region

1. Which is the material under test ? (identifying unknown material)

2. What Functional Groups are Present in the

material under test ?

How fingerprint region is important in identifying unknowns ?

04/18/2023 21

Case Study : Convergent synthesis of Alkenes by Cross Coupling of Carbenes

Carbenoid

Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L.; Angew. Chem. Int. Ed., 50: 2544–2548

22

Convergent synthesis of Alkenes by Cross Coupling of Carbenes


Acceptor carbenoid (2) homodimerization is very favorableDonor/acceptor carbenoid (1a) homodimerization is not favorableWhat factors control the cross coupling and why does it work so

well?

Donor/Acceptor + Acceptor

23

In-situ Analysis

(1a) decomposes faster than the (2) and then preferentially reacts with other diazo compound.

Reaction monitored by the presence of C=N stretch bands


Acceptor Donor/Acceptor

24

Cross Coupling of Carbene In Equimolar mixture of (1a) and (2), the

consumption of (2) is 1.5 times faster than (1a) All of 2 is consumed within 7s Once 2 is consumed the decomposition of 1a is

significantly low. Homodimerization of (2) does not go to

completion


Acceptor Donor/Acceptor

25

Mechanism for Cross coupling of Carbene


Product cross dimerization rely on:

A significant decomposition

rate difference between the

two diazo compounds

A preference for trapping of

the initial carbenoid with

the other diazo compound.

Key findings:

04/18/2023 26

SummaryProvides a full kinetic analysis from a

minimum number of reaction progress experiments

Elucidation of mechanism and pathway

Detection of reactive intermediatesOptimize reactions and develop safe ways

to work with carbenoid precursors

THANKS FOR YOUR ATTENTION

Spectroscopy Presention

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