Catalysis

using

metal organic frameworks

Synopsis Report for M.Tech (Research) Thesis

Submitted by

Under the guidance of

Dr. Pradip Chowdhury

DEPARTMENT OF CHEMICAL ENGINEERING

NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA

DECEMBER 2012

Prince George

610CH306

The aim and scope of present work can be broadly categorized into following sections:

A. Oxidative degradation of polystyrene, Selection criteria for synthesized MOFs (based

on thermal stability).

B. Photocatalytic degradation of organic dyes (basic and acidic), Selection criteria for

synthesised MOFs (based on aqueous stability and band gap).

The complete thesis is organized into following chapters:

Chapter 1 Introduction

Chapter 2 Literature Review

Chapter 3 Experimental Methods

Chapter 4 Results and Discussion

Chapter 5 Future Scope of Work and Conclusion

Each of the chapters is classified into various sections and sub-sections accenting and

supporting relevant subject matters.

Chapter 1 Introduction

The chapter directs our vision on using metal organic frameworks as catalysts for the

degradation and handling of various non-environment friendly materials. The definition,

chemistry of synthesis, the advantages and disadvantages associated with MOFs are also

discussed. Finally, the basis for our research to use MOFs and the objectives that are focused

in for research are stressed.

Background of present day research

Metal organic frameworks are porous crystalline solids composed of three dimensional

network of metal ions held by organic ligands. The arrangement of such leads to a system of

channels and cavities analogous to zeolites. Alongside there are many salient features such as

exceptionally high specific area (say 1000-5000 m2/g ),uniform pore size distribution, low to

moderate thermal stability, tuneable pore size etc. Many of these properties are now finding

applications in the areas of gas separation, storage. MOFs contain large percentage of metal

ions that can act as catalytic sites provided the metal ions have free co-ordination sites. Due

to similarities with zeolites, MOFs can find applications in the area of solid catalysts. The

main drawbacks of MOFs being compared to zeolites are lower thermal and chemical

stability. Another such area of latest research is the application of MOFs as semiconductor

catalysts. Many MOFs have transition metals as structural nodes and that they are separated

by the organic linkers, analogous conduction bands and effective hybridization of vacant d

orbitals with molecular orbitals of organic links. Latest studies have shown promises in

design and tuning of suitable band-gaps of various MOFs by varying in organic and organic

linkers through computational and experimental studies.

One such major application that is focussed in this report is the thermal degradation of

polystyrene, a polymer of styrene that usually starts degradation at temperature of 300-

350C.There are various environmental effects that can be brought about due to improper

degradation of polystyrene as it is biologically inert having infinite half-life in the natural

environment.

Another area of application focussed is the Photocatalytic degradation of dyes. The two dyes

are taken under consideration basic dye, Crystal Violet; acidic dye, Coomassie Blue R-125.

Both dyes have various applications in textile and as a biological stain or dermatological

agents. In spite of extensive applications Crystal violet, Coomassie blue, are mitotic

poisoning agents and are considered as hazardous substances that are stable compounds

resistant to degradation.

Objectives

The main objectives our present research work can be summarized as follows:

A. Synthesis, characterization and selection of a suitable metal organic frameworks or MOFs, effective for catalytic applications.

B. Oxidative degradation of polystyrene using metal organic frameworks(MOFs) i. Comprehensive study of oxidative degradation of polystyrene using

synthesised MOFs.

ii. Evaluating the best MOF for effective breakdown of polystyrene and optimum

catalyst (MOF) to polystyrene ratio.

iii. Estimation of kinetic parameters such as order and activation energies.

C. Photocatalytic degradation/decolourization of dyes. i. Comprehensive study of degradation/decolourization of Crystal Violet and

Coomassie Blue R-125 using synthesised MOFs.

ii. Evaluating the best MOF and combination for effective

degradation/decolourization of said dyes.

iii. Estimation of kinetic and interaction parameters involved in

degradation/decolourization.

Chapter 2: Literature Review

Comprehensive literature reviews on MOFs with special focus on applications are discussed

in detail. The underlying principles of their synthesis and structural geometry are also

elaborated. A chronology of recent events in the field of thermal degradation of polystyrene

and degradation of dyes under study are also detailed [1-9].

Chapter 3: Experimental Methods

Synthesis of MOFs

Four different MOFs were synthesized viz. Cu-BTC or HKUST-1, following the procedure

explained by Chui et al; Zn-BDC or MOF-5, following the method of Jinping Li et al[10],

Fe-BDC or MIL-53(Fe), following the method by Fery et al[11]. A novel Pb-BTC was

synthesised by following the procedure explained by Chui et al by altering the inorganic

ligand with Lead(Pb) instead of Copper(Cu). Furthermore alterations were made in the

stoichiometry of Fery et al [11]; for the synthesis of Fe-BDC or MIL-53(Fe) dopant with

Lithium (Li).

The hydrothermal route was followed for all the synthesis of MOFs following certain

common steps involving maintaining a standard stoichiometry, pre synthesis treatments (i.e.

proper mixing and stirring) followed by transferring the final solution into a Teflon lined

stainless steel autoclave, allowing the reaction to occur at a standard temperature and duration

of reaction was also closely monitored. Post synthesis treatments were found to be very

crucial in determining the final outcome and the quality of the reaction product. Each and

every step in synthesizing MOF crystals were carefully repeated for streamlining them into a

standard recipe. Processed MOFs were then sealed in air tight acrylic containers were stored

in a desiccator.

Characterization

Characterization of MOF powder samples were carried out systematically. The prominent

characterization techniques that were adopted for analysis were SEM, XRD, BET and TGA.

The images, for studying the MOF crystals were taken via scanning electron microscope

(JEOL JSM-6480 LV) equipped with an energy dispersive X-ray spectrometer (EDX). The

samples were gold-coated for a better conductivity, helping the imaging process. The

crystalline phase of the MOF powders were determined using an X-ray diffractometer

(Philips Analytical, PW-3040) equipped with the graphite monochromatized Cu K radiation

(=1.5406 ) in 2 angles ranging between 5o

reflectance spectroscopy by UV-Vis Spectrometer SHIMADZU (UV-Vis ).MOFs with lower

band gaps were chosen for photo catalytic experiments. Photocatalytic experiments were

carried out in three modes,

i. In the dark (Reference)

ii. In sunlight

iii. In ordinary light (provided by High pressure Hg vapour lamp,100W)

The studies were carried out on two different dyes: basic dye, Crystal violet; acidic dye,

Coomassie Blue R-125.The experimental setting for determination of photo catalytic

degradation kinetics is tabulated below Table 3.2.

Conditions Dye (CyV,CoB) Catalyst(MOFs) Enhancer (H2O2)

Dark

Light

Table 3.2 Experimental pattern for determination of kinetics

The degradation effects were studied under different parameters for both dye combinations.

The parameters considered were pH, concentration of dye, weight of catalyst and

concentration of enhancer. Photocatalytic degradation was studied by observing the change in

absorbance of dye solution during the reaction using Jasco (V-530) UV/Visible

spectrophotometer in the visible range (400-700nm).

Chapter 4 Results and Discussion

For studying the catalytic application of synthesised MOFs, the materials synthesised were

characterised after post synthesis treatment. Initially four different MOFs were synthesised,

and characterised using SEM imaging, PXRD, BET surface area, and TGA. Material

characteristics from the above analysis were inferred and a suitable catalytic application was

selected. The characterization data of synthesised materials are given below.

Figure 4.1: SEM images of MOFs :( A) Cu-BTC (or, HKUST-1) (B) Fe-BDC (or, MIL-53(Fe)) (C) Zn-BDC

(or, MOF-5) (D) Pb -BTC

Figure 4.2: PXRD data of MOFs :( A) Cu-BTC (or, HKUST-1) (B) Fe-BDC (or, MIL-53(Fe)) (C) Zn-BDC (or,

MOF-5) (D) Pb -BTC

Figure 4.3: TGA profile of MOFs Table 4.1: BET surface area of MOFs

From the above data, it can be inferred that the materials (MOFs) synthesized are crystalline,

moderately/highly porous and has high temperature instability. Therefore, synthesized MOFs

can be used for applications that have temperature less than the breakdown temperature of

MOFs. For most MOFs absorb moisture, the initial weight loss in the TGA profile was due to

the loss of moisture. The plateau region in the profile is the temperature range suitable for

any catalytic reactions, above the breakdown temperature the structure of MOF collapses.

From SEM, PXRD and BET surface area it was inferred that the materials synthesized are

MOFs, as the observations tallies with the literature data available.

Slno Material (MOF) BET surface area (m2/g)

1 Cu-BTC (HKUST-1 1492

2 Fe-BDC (MIL-53(Fe) 360.06

3 Zn-BDC(MOF-5) 856.3

4 Pb-BTC 11.28

B

D

C

A

B

D

C

A

Photocatalytic degradation/decolourization of organic dyes

Photocatalytic degradation/decolourization is governed by two fundamental parameters,

solvent stability and band gap of said MOF. Initially in the aqueous stability was inferred to

be a failure for Cu-BTC; Zn-BDC (MOF-5) from the literature report[] fails its stability in

water. Only MIL-53(Fe) Fe-BDC passed aqueous stability test. The band gaps MIL-53(Fe)

and its dopants were calculated.

Slno Material (MOF) Wavelength Band-gap

nm e V

1 MIL-53(Fe)-washed 517.5 2.4

2 MIL-53(Fe)-1% Li 496.8 2.5

3 MIL-53(Fe)-10% Li 552.5 2.25 Table 4.2 Band gap voltage of MIL-53(Fe) & its dopants

From the above Table 4.2, It can clearly be understood that MIL-53(Fe) and its dopant at

higher concentration has less band gap wavelength falling in the near UV-visible region

henceforth high pressure mercury vapour lamp (100W) was chosen as the source of artificial

white light.

The dyes used for photo catalytic reaction were Coomassie Blue and Crystal Violet. The first

dye for degradation was Coomassie blue, an acidic dye concentraction about 20 ppm

maximum. The figures below indicate the degradation profile of said dye in artificial white

light (100W).

B

C

A

Figure 4.6: Degradation kinetics profiles

(A),(B),(C) Combined degradation profile, Coomassie

Blue with varying MIL-53(Fe) Wt. at 4 ,7,9 p H

respectively.

The reaction equations was fitted to zero and first order kinetics, following kinetic constants

were tabulated in Table 4.3 for varying p H conditions.

Slno p H Order kavg Catalyst Wt.(g)

1 4 0 0.0052* 0.1

2 4 0 0.0061* 0.2

3 7 0 0.0048* 0.1

4 7 0 0.005* 0.2

5 9 1 0.010742# 0.2 Table 4.3 Kinetics data for degradation of Coomassie Blue (units * molmin-1 and # min-1)

From the above data it can be inferred at higher concentration of dye, the order of kinetics

was zero and when concentration is small, the order was first order. The interaction of

enhancer was also considered in the kinetics.

The maximum degradation percentage was observed for two different conditions of p H (i.e.

4.0 and 9.0 about 68% for both. The degradation profile with time can be observed from

Figure 4.7

Figure 4.7: Degradation kinetics profiles (A) & (B) for the entire spectrum detailed to Figure 4.6 (A) and

(C) respectively.

MIL-53(Fe) doped with 10% Li as photo catalyst for degradation

Figure 4.8: Degradation kinetics profile Figure (A) at 9.0 p H and Figure (B) the entire spectrum detailed.

B

A

B

A

The presence of enhancer alone does not effect in the degradation of the said dye (say

maximum degradation about 9 %).From the above degradation profile it can inferred from the

kinetics that the order of reaction shifts from zero order to first order with decrease in dye

concentration and also by increase in catalyst loading. The pH drastically effects the

degradation in both cases of pure MIL-53(Fe) and the doped MIL-53(Fe). 10% Li doped

MIL-53(Fe) partially proved promising in the absence on enhancer, the degradation

percentage was about 41% at 7 p H, and in contrast to 43.7% in presence of 10% Li doped

MOF and enhancer concentration of 0.1m M.

Comparing Figure 4.7 (B) and Figure 4.8(B), it can be observed that change in degradation

profiles are similar, but best degradation is brought about by MIL-53(Fe).

Photo catalytic degradation /decolourization of Crystal Violet

Crystal violet is one of the basic dyes, from experimental observation it can be inferred that at

higher concentration of enhancer (1m M) in a system of 40ppm brings about complete

degradation irrespective of the catalyst used. Under varying p H environment degradation

studies was carried out.

Figure 4.9: Degradation kinetics profiles

(A),(B),(C) Combined degradation profile, Crystal Violet with varying MIL-53(Fe) Wt. at 4 ,7,9 p H

respectively.

From the Figure 4.9, inference can be drawn initially the concentration of the dye drops due

to the presence of hydroxyl radicals in system, further on depletion of free radical tends to

formation reaction intermediates that can observed from the change in wavelength of

maximum absorbance. After, a particular amount time degradation proceeds with the change

in colour of the dye solution from violet to pink and then the intensity of pink fades out to

colourless. For lower concentration of dye, enhancer and catalyst at p H 9.0 the combination

B

A

C

has synergic effect as in Figure 4.9(C), while under all other cases the kinetics is mere

additive as can be observed from Figure 4.9 (A&B).

Slno p H Order kavg Conc Enhancer (m M) Catalyst Wt.(g)

1 9 1 0.003* 0.01 0.0

2 9 1 0.01* 0.1 0.2 Table 4.4 Kinetics data for degradation of Crystal Violet (units * min-1)

Table 4.4 shows the estimated kinetic parameters for optimum degradation of crystal violet

with least concentration of enhancer, to bring about the best degradation about 62.2% with

synergic index 0f 2.5.

MIL-53(Fe) doped with 10% Li as photo catalyst for degradation

Figure 4.10: Degradation kinetics profile Figure (A) at 7.0 p H and Figure (B) the entire spectrum detailed.

From the Figure 4.10, the best degradation obtained at neutral p H was about 52.63% and

follows first order kinetics with kavg 0.02447 min-1 .From the Figure 4.10(A) synergic index

was clearly calculated to be 1.4. Comparing Figure 4.9(C) and 4.10(A), it can clearly inferred

that MIL-53(Fe) 10% Li doped MOF was found far effective than regular MIL-53(Fe).

Oxidative degradation of polystyrene

One such applied application was to study the oxidative degradation of polystyrene in

presence of MOFs. Constrain was applied is on the temperature due the reason explained

above, as polystyrene starts to undergo thermal degradation above 350 C. The breakdown

temperature of MOFs can be referred to Table 3.1.The reaction kinetics was carried out only

in the narrow range of temperature due to instability of MOFs above their breakdown

temperature. The region of kinetics under study is the plateau region in the MOF degradation

profile, where any change in degradation of the material can readily observed and quantified

B

A

with the influence of other influences such as moisture, solvent traces. The degradation

kinetics was studied and the following details can be inferred from the graphs below.

From Figure 4.4 (A), it can be inferred, that of the four MOF polystyrene mixtures the best

of the two are Zn-BDC and Pb-BTC, so Cu-BTC, Fe-BDC are rejected for further studied.

Cu-BTC, the weight loss can be incorporated towards the loss of water vapour/moisture from

lattice alongside with the degradation of the said polymer. The thermal stability of Cu-BTC

was found to be the least of all the synthesised MOFs. The rejection of MIL-53(Fe) is due to

its relative short temperature range of stability and active polystyrene degradation occurs only

after 300 C.

Figure 4.5: (A) Degradation percentage with different best combination ratios of MOFs with polystyrene.

(B) Activation energy for different best combination ratios of MOFs with polystyrene.

Figure 4.4: Degradation kinetics profiles

(A) Combined degradation profile, with

polystyrene and MOFs at 50-50 wt. %.

(B)Combined degradation profile, with

polystyrene and Zn-BDC at different weight

ratios.

(C) Combined degradation profile, with

polystyrene and Pb-BTC at different weight

ratios.

B

C

A

B

A

Figures 4.4 (B) & (C) clearly shows the acceleration in oxidative degradation in pure

polystyrene in presence of Zn-BDC and Pb-BTC, shifts from 415 C to around 320 C. The

nature of the kinetics for both Zn-BDC and Pb-BTC are similar on observing the degradation

profile. First, pure polystyrene melts above 150 C and on further increase in temperature

burns in air; in case of Zn-BDC due to high surface area provides more contact with air for

the oxidation of the polymer. It can be inferred from Figure 4.4 (C) Pb-BTC surface area does

not play in the degradation of polystyrene, but on nature of Pb-BTC.

From Figure 4.5 (A) it can be observed that higher percentage of degradation was attained at

50-50 combination for Zn-BDC, and then drops with increase in ratio of polymer is mainly

due to decrease in active surface area for the catalyst, in case of the degradation is not

influenced by surface area of the catalyst as the percentage of degradation almost remains

constant.

Kinetics of degradation was calculated using Kofstard method [12] and activation energy was

calculated, the activation energy of pure polystyrene agrees with the literature data of

80KJ/mol (i.e. 75.74KJ/mol).Using Pb-BTC 50% drops the activation energy about 68.1%,

similar followed by Zn-BDC 30% about 68%.The degradation percentage and activation

energy does not fluctuate much in case of Zn-BDC is on average about 33.62% and

22.57KJ/mol respectively. The Zn-BDC that can be employed for polystyrene degradation

was found to be Zn-BDC with best combination at 50%wt, but for mixture composition of

Pb-BTC and polystyrene (i.e. 30/70, 10/90) the conversion almost remains uniform and

relative has higher edge to Zn-BDCs mixture combination at lower ratios on comparing

catalyst weight percentage to degradation percentage.

Chapter 5 Future Scope of Work and Conclusion

To conclude, totally five different MOFs were synthesised of which two are novel MOFs and

have shown promising results in different applications such as oxidative degradation of

polystyrene and photo catalytic degradation of dyes. Doping of MIL-53(Fe) with Lithium

was successful and reduction in band gap energy was achived from 2.4 e V to 2.25 e V for

10% doped Lithium.

In polystyrene degradation, in different combinations both Zn-BDC (MOF-5) and Pb-BTC

has shown degradation about 37% and 34% respectively. With decrease in degradation

temperature about 95 C to 90 C for both combinations of polystyrene /MOF mixtures. The

best combination was found out to be polystyrene-Zn-BDC 50-50 Wt. %.

For the photo catalytic degradation of dyes, In case of Coomassie Blue R-125, 10% Li doped

MIL-53(Fe) partially proved promising in the absence on enhancer, the degradation

percentage was about 41% at 7 p H, and in contrast to 43.7% in presence of 10% Li doped

MOF and enhancer concentration of 0.1m M. While best case of degradation was observed at

4 p H about 69.4% with enhancer concentration of 0.1 m M.

In case of Crystal Violet, 10% Li doped MIL-53(Fe) has proved promising in presence of

enhancer concentration 0.1m M with degradation of 52.63% with lower synergic index of

1.4, while regular MIL-53(Fe) showed degradation of 62.2% in presence of enhancer

concentration of 0.1 m M with higher synergic index of 2.5.

The degradation of polystyrene is to be analysed to determine the nature and mechanism of

breakdown and products formed during low temperature oxidation. Scaling the process to

pilot plant and studied has to carried out on catalyst recyclability.

In section of dye degradation, scale up of the process using kinetics data to pilot scale plant

for more effective degradation using sunlight as light source has shown promising results.

Studies on catalyst recyclability have to be evaluated to determine the lifetime of MOFs after

the reaction. Immobilizing MOFs on a transparent film /glass to determine the effectively of

said system can open new venues in detection and diagnostics research.

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