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Large-ring aromatic sulfur compounds e.g. benzothiophene, dibenzothiophene present in petroleum and liquid fuelsÂ are harmful to our health and environment. They cause acid rain due to SO2 formations upon combustion of liquid fossil fuels. We studied advanced materials Metal Organic Frameworks, MOFs, e.g. Cu-MOF Basolite C300 Cu3(C9H3O6)2 to remove aromatic sulfur compounds from model liquid fossil fuels (solution in tetradecane) selectively and in a non-destructive fashion. Kinetics of adsorption of aromatic sulfur compounds on Cu-MOF and Fe-MOF was investigated under ambient conditions. Adsorption capacity of Cu-MOF vs. large-ring aromatic sulfur compounds was examined under thermodynamic equilibrium at different temperatures. Chemical analysis of liquid phase was performed by UV/VIS spectroscopy (quantitative determination) and high performance liquid chromatography HPLC (qualitative chemical analysis). Quantum chemical simulations of the structure of aromatic sulfur compound DBT and its molecular spectra were conducted. Evidence of molecular interaction between MOFs and aromatic sulfur compounds was shown by fluorescence spectroscopy.
ACKNOWLEDGEMENTS AND DEDICATION
I would like to express my gratitude to Dr. AlexanderÂ Samokhvalov, who was my M.Sc. thesis advisor, for his ideas and guidance on this entire dissertation. I would also like to thank my M.Sc. Committee members: Alex J. Roche and George Kumi for their help with editing my dissertation.
I am grateful to Turkish government for funding my graduate work. In particular, I would like to thank Turkish Educational Attaché at New York for their constant support and mentorship.
I thank the Department of Chemistry Faculty, Staff, and fellow Graduate Students whom I had the pleasure to work with during my Graduate studies at Rutgers University.
This Research and work for the Master Dedicated to:
My father Temo DEMIR
Without your sacrifices, this would not have been possible. Thank you for constantly supporting me throughout this process.
My brother Yasin.
Thank you for pushing me to do my best at an early age. The values you have instilled in me have made this journey possible.
My brother Sait
I hope this accomplishment in my life demonstrates to you what can be achieved with perseverance, dedication and hard work. You were my inspiration through this.
TABLE OF CONTENTS
ABSTRACT OF THE DISSERTATION i
ACKNOWLEDGEMENTS AND DEDICATION ii
LIST OF FIGURES iv
LIST OF TABLES v
CHAPTER 1: INTRODUCTION 1
1.1 FOSSILS, PETROLIUM AND CLEAN FUELS 1
1.2. SULFUR AROMATIC COMPOUNDS 3
1.3 METHODS OF DESULFURIZATION OF LIQUED FUELS 5
1.3.1 HYDRODESULPHURIZATION 6
1.3.2 PHOTOOXIDATION 6
1.3.3 OXIDATIVE DESULFURIZATION 7
1.3.4 BIODESULFURIZATION 7
1.4 ADSORPTION OF AROMATIC SULFUR COMPOUNDS FROM LIQUID PHASE 8
1.5 METAL-ORGANIC FRAMEWORKS MOFS 9
1.6 ADSORPTION OF AROMATIC SULFUR COMPOUNDS ON MOFS 11
1.7 RESEARCH OBJECTIVE 11
CHAPTER 2: EXPERIMENTAL 12
2.1 SULFUR AROMATIC COMPOUDS 12
2.2 MODEL FUELS 12
2.3 ACTIVATION OF MOFS BEFORE ADSORPTION 12
2.4.1 UV/VIS SPECTRA OF AROMATIC SULFUR COMPOUNDS 14
2.4.2 CALIBRATION CURVE OF AROMATIC SULFUR COMPOUNDS 14
2.5 ADSORPTION AT CONSTANT TEMPERATURE 14
2.6 ADSORPTION AT VARIABLE TEMPERATURE 16
2.7 CHEMICAL ANALYSIS BY HPLC-UV (HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY) 17
2.8 FLUORESCENCE SPECTRA OF MOFS AND SULFUR AROMATIC COMPOUNDS 18
2.9 QUANTUM MECHANICAL CALCULATIONS OF STRUCTURE OF REPRESENTATIVE AROMATIC SULFUR COMPOUND 18
CHAPTER 3: RESULTS AND DISCUSSION 20
3.1 UV/VIS SPECTRA OF AROMATIC SULFUR COMPOUNDS AND UV/VIS CALIBRATION CURVES 20
3.2 CHEMICAL COMPOSITION OF MODEL FUELS BY HPLC-UV AFTER ADSORPTION 25
3.3 KINETICS OF ADSORPTION AT CONSTANT TEMPERATURE 30
3.4 TEMPERATURE-PROGRAMMED ADSORPTION/DESORPTION OF AROMATIC SULFUR COMPOUNDS ON MOF 31
3.5 STOICHIOMETRY OF ADSORPTION COMPLEXES 37
3.6 RAMAN AND FLUORESCENCE SPECTRA OF ADSORPTION COMPLEXES 38
3.7 COMPUTATIONAL RESULTS 40
CHAPTER 4: APPENDICES 45
CHAPTER 5: REFERENCES 46
LIST OF FIGURES
Figure 1: Aromatic sulfur compounds from exhaust cause air pollution. 2
Figure 2: Aromatic sulfur compounds presents in fuels 4
Figure 3 : Structure of Cu-MOF . 10
Figure 4: Apparatus for activation of MOFs 13
Figure 5: Apparatus of adsorption/desorption bounded to shaker 14
Figure 6: Sequence of heating and sampling for analysis 17
Figure 7: UV/VIS Spectra of Benzothiophene in solution of n-C14H30 20
Figure 8: UV/VIS Spectra of solution of Dibenzothiophene n-C14H30 22
Figure 9: UV/VIS spectra of solution of 4,6-Dimethyldibenzothiophene n-C14H30 23
Figure 10: Calibration curve for quantitative determination of BT by UV/VIS spectroscopy 24
Figure 11: Calibration curve for quantitative determination of 4,6-DMDBT by UV/VIS spectroscopy 25
Figure 12: HPLC-UV spectrum of BT onto Cu-MOF at 299 nm 26
Figure 13: HPLC-UV time-absorbance 3D trace for BT in C14H30 27
Figure 14: HPLC-UV spectrum of DBT onto Cu-MOF at 326 nm 28
Figure 15: HPLC-UV time-absorbance 3D trace for DBT in C14H30 29
Figure 16: Kinetic of adsorption of DBT on Cu-MOF at constant temperature at 0.033M 30
Figure 17: Extent of sulfur compounds adsorption at equilibrium under 298, 348 and 388 K 32
Figure 18: Adsorption/desorption cycle of Thiophene 33
Figure 19: Adsorption/desorption cycle of BT 34
Figure 20: Adsorption/desorption cycle of DBT 35
Figure 21: Adsorption/desorption cycle of 4,6 DMDBT 36
Figure 22 : Fluorescence spectra of solid suspension of Cu-MOF in BT 38
Figure 23: Gaussian view of DBT 40
Figure 24: IR spectrum of DBT with Gaussian programming 43
Figure 25: UV-VIS spectrum of DBT with Gaussian programming 44
LIST OF TABLES
Table 1: Sulfur levels for gasoline and diesel  3
Table 2: Physical properties of some sulfur aromatic compounds 5
Table 3: Literature data on surface area, pore size and pore volume of sorbent  10
Table 4: Stoichiometry ratio of Cu-MOF toward to aromatic sulfur compounds. 37
Table 5: Bond lengths and Angles: Hartree-Fock level of the theory using the standard 6-311** basis set for optimization 41
Table 6: Bond lengths and Angles: the DFT (B3LYP) level of theory using the standard 3-21G** basis set 42
CHAPTER 1: INTRODUCTION
1.1 FOSSILS, PETROLIUM AND CLEAN FUELS
The world takes many rigorous steps to standardize sulfur emissions. There are many reasons for lowering sulfur levels in liquid fuels. Firstly, combustion of sulfur compounds present on liquid fuels produces harmful gases which result in acid rain via reaction with water forming sulfurous and sulfuric acids. These acid rains are harmful to our health systems, environment, and economy. Acid rain is corrosive and causes damage to plants, roofs of homes, cars, trucks, metal structures, and cement. Moreover, acid rain irritates several organs such as the heart and causes many illnesses and diseases such as asthma and bronchitis. Aromatic sulfur compounds can contaminate water and soil sources. Furthermore, these sulfur compounds contributes to chemical substances which cause smog [ [i] ].
An emission control system of trucks and cars would require low amounts of sulfur compounds in liquid fuels [ [ii] ]. Before any removal of sulfur compounds from crude oil, total sulfur levels can be in the range of 100 and 33,000 ppm. For all above reasons, the EPA (Environmental Protection Agency) has certain limitations on the sulfur levels present in petroleum. The USA legitimate levels of total sulfur in transportation fuels must not be higher than 15 ppmw for diesel fuel and not higher than 30 ppmw for gasoline. For some applications e.g fuel cells, 'zero sulfur' fuels which require ultradeep desulfurization are needed with total sulfur content < 1 ppmw [ [iii] ]. According to the EURO IX standard, sulfur content is required to be less than 50 ppmw in diesel fuel for most highway vehicles [ [iv] ]. However, the ULSD (ultra low sulfur diesel) is now required to contain a maximum of 10 ppmw total sulfur, similarly to the new Euro V standard of 2009 .
Figure 1: Aromatic sulfur compounds from exhaust cause air pollution.
Several criteria must be considered in the ultra-desulfurization of liquid fuels. The first one is capital cost, in which lower unit operation, minimum equipment, and cheaper materials are taken into account in order to keep prices inexpensive. The second one is operation cost in which the use of hydrogen and generation of waste should be minimized. The product volume capacity must be as high as 99 percent and the process cycle life should be sufficiently long enough; also, the operation system should be simple and use the least amount of catalysts [ [v] ].
Table 1: Sulfur levels for gasoline and diesel [ [vi] ]
Sulfur limit (ppm)
1.2. SULFUR AROMATIC COMPOUNDS
Heteroaromatic sulfur-containing organic compounds such as alkyl-substituted thiophene, benzothiophenes (BT), and dibenzothiophenes (DBT) are the abundant components of fossils and fossil fuels such as petroleum [ [vii] ], oil shale [ [viii] ], tar sands [ [ix] ], bitumen [ [x] ]. The content of sulfur in crude petroleum is between about 100 [ [xi] ] and 80,000 ppmw [ [xii] ].
Figure 2: Aromatic sulfur compounds presents in fuels
Many aromatic sulfur compounds can be found in products of petroleum refining and upgrading: napthas [ [xiii] ] and gas oils. Aromatic sulfur compounds are also present in commercial products of processing of fossils such as gasoline [ [xiv] ], diesel [ [xv] ], and jet fuels. With significant recent developments, molecular structure and concentration of representative aromatic sulfur compounds have been determined in petroleum [ [xvi] ], refinery naphthas [ [xvii] ], oils [ [xviii] ], and commercial liquid fuels [ [xix] ]. Heteroaromatic polymers present in brown coals have their major structural units similar to those of aromatic sulfur compounds [ [xx] ]. Thus, coal lique faction produces liquid fuels that contain high concentration of aromatic sulfur compounds [ [xxi] ]. When fossil fuels containing sulfur compounds are burned, sulfur is released as sulfur dioxide SO2 which is a toxic and corrosive substance [ [xxii] ], and as direct particulate matter (DPM) containing sulfur [ [xxiii] ]. Once in the air, sulfur compounds pose a serious health hazard and cause malfunctioning of all major pollution control technologies, such as automobile catalytic converters [ [xxiv] ]. Similarly, incomplete burning of transportation fuels releases toxic sulfur aromatic compounds into the air [ [xxv] ]. The liquid waste of refineries also contains a significant concentration of aromatic sulfur compounds. The major source of contamination by sulfur aromatic compounds is crude petroleum spills [ [xxvi] ], e.g., the recent oil leak in the Gulf of Mexico [ [xxvii] ]. Some of the most harmful and toxic compounds found in marine environments include methyl-substituted sulfur aromatic compounds [ [xxviii] ]. Aromatic sulfur compounds are also present outside of fossil fuels in molecules of certain organic semiconductors , pesticides , drugs , components of the soil , and human body (pheomelanin pigments) [ [xxix] xxxxxxixxxii].
Calculations of fundamental thermodynamic properties of aromatic sulfur compounds, such as molar enthalpies of formation and standard molar enthalpies of phase transitions, can be found in several quantum chemistry papers [ [xxxiii] ].
Table 2: Physical properties of some sulfur aromatic compounds
Aromatic sulfur compounds
Boiling point(K degree)
Solubility on Tetradecane
1.3 METHODS OF DESULFURIZATION OF LIQUED FUELS
There are several methods to remove sulfur compounds from fossil fuels such as gasoline, diesel, and jet fuels. Some of these methods have been already investigated well and used industrially, e.g. conventional hydrodesulphurization (used in industrial scale at refineries).
However, hydrodesulfurization is not effective enough for ultra-deep desulfurization of fuels containing large amounts of the large-ring aromatic sulfur compounds. Alternative methods need to be studied and developed using novel types of sorbents.
Desulfurization of petroleum to produce clean liquid fuels such as gasoline or diesel via catalytic hydrodesulphurization (HDS) is well known used in industry [ [xxxiv] ]. Most of sulfur compounds such as thiols, thiolates, sulfoxides and sulfones are removed by hydrodesulphurization at high temperature and pressure. Industrial method to remove aromatic sulfur compounds from fossil hydrocarbon fuels by catalytic hydrodesulfurization (HDS) is 1) not effective in ultradeep removal of large-ring sulfur aromatic compounds below about 15 ppmw total sulfur, 2) not very selective for sulfur aromatic compounds versus aromatic compounds such as e.g. naphthalene, due to chemical resistance of large aromatic rings and 3) energy demanding and not CO2 neutral. Many alkyl-substituted such as benzothiophenes, dibenzothiophenes, and benzonaphthothiophenes are refractory in HDS process [ [xxxv] , [xxxvi] ]. The mechanism of refractory behavior of large-ring aromatic sulfur compounds via HDS is sterical hindrance effect [ [xxxvii] ].
Heterogeneous photocatalysis were utilized for environmental applications such as purification of water and air via photocatalytic oxidation [ [xxxviii] ]. Recently, there has been a strong growth of interest towards photocatalysis such as photocatalytic water splitting , photocatalytic CO2 reduction [ [xxxix] ]. From a practical standpoint, heterogeneous photocatalysis of aromatic sulfur compounds is of relevance to petrochemistry [ [xl] ], emerging "ultraclean" fuels for fuel cells, and environmental research and applications [ [xli] ].
1.3.3 OXIDATIVE DESULFURIZATION
Among catalysis-based methods of removal of aromatic sulfur compounds, catalytic oxidation is of major interest. Oxidative desulfurization has been used particularly for diesel desulfurization. In this method, sulfur compounds converted to oxidized sulfur species via oxidative chemicals such as HnO3,NO/NO2,RuO4.In contrast to HDS that produce H2S, Oxidative desulfurization process removes oxidation products by solvent extraction. Oxidative desulfurization proceeds at mild conditions and not need molecular hydrogen gas H2 . Major disadvantages of desulfurization via catalytic oxidation are the use of expensive chemicals, (organic peroxides and H2O2), the use of highly corrosive or oxidative liquid media, and the need to dispose chemical waste .
Another active method for removing aromatic sulfur compounds from liquid fuels is bio desulfurization. Sulfur is necessary element for some microorganism to grow and sustain for life [ [xlii] ]. So, microorganism can be used l to convert aromatic sulfur compounds into oxidized focus of sulfur which easily removal compounds. Biodesulfurization process has been used to remove sulfur compounds under mild condition, via a non-invasive approach, and recently found application in industrial desulfurization [ [xliii] ].
1.4 ADSORPTION OF AROMATIC SULFUR COMPOUNDS FROM LIQUID PHASE
Adsorption of aromatic sulfur compounds via ion exchange zeolite and metal halide-impregnated carbon is a promising desulfurization method to remove sulfur selectively from liquid fuels [ [xliv] xlv]. There are many specific advantages using adsorption process to remove sulfur from petroleum and gasoline. Firstly, adsorption process occurs at ambient conditions (room temperature and atmospheric pressure) which make it a low cost process. Secondly, adsorption process does not have to consume any oxygen and hydrogen. Many adsorbents such as zeolites (mesoporous materials) and activated carbon have been investigated for desulfurization. Ma and Yang have researched using ion exchanged zeolite and metal halide-impregnated carbon for desulfurization of liquid fuels [ [xlvi] ]. These researches showed that adsorption capacity of metal halide-impregnated carbon for refractory aromatic sulfur compounds is higher than that of ion exchanged zeolites. Kim et al. has analyzed metals such as Ni deposited on silica gel, activated aluminum and activated carbon to remove refractory aromatic sulfur compounds and aromatic nitrogen compounds selectively[ [xlvii] ]. This research also indicated that adsorption capacity of metal halide - impregnated activated carbons is related to surface area, total pore volume, BET surface area which is directly proportional to adsorption capacity of desulfurization and denitrogenation. These results are consistent with the microspore diameter and dimensions of both aromatic sulfur compounds and metal cation which is of critical importance for adsorption of sulfur compounds, particularly aromatic compounds [ [xlviii] ].
1.5 METAL-ORGANIC FRAMEWORKS MOFS
Metal-Organic Framework (MOFs) are the new class of metal-organic polymers which are known for their very high adsorption capacity, particularly of hydrogen at moderate operation conditions. The structure of MOFs is formed by metal site bound to organic ligand thus forming three dimensional polymer networks, which are of inorganic-organic hybrid type. Bonding occurs between metal ions and organic linkers, the latter acting as bridging ligands between the metal ions to compose a 3D structure. Number of possible structures of MOFs is virtually infinite due to the variety of available subclasses of MOFs [ [xlix] , [l] , [li] , [lii] ].
MOFs are promising materials for industrial usage such as for gas storage, separation, sensing, catalysis and adsorption of unwanted species [39, 40, 41, and 42]. MOFs have many great properties such as large pore volume and high inner surface area. Moreover, host-guest interaction of MOFs with molecules of adsorbate makes MOFs very important for applications. Although investigation of MOFs for adsorption of aromatic sulfur compounds such as BT, DBT has been reported [ [liii] ], industrial usage of MOFs as a sorbent has not occurred. One of the well-investigated MOFs is HKUST-1 that is also known as Basolite C300 Cu-MOF. The structure of Cu-MOF known as Basolite C300 called HKUST-1 compounds is formed by metal site bound to organic ligand forming 3D polymer network. Figure 3 shows the structure of Cu-MOF with formula Cu3(btc)2. Grey balls indicate Carbon, red balls indicate Oxygen, and purple balls indicate Cu2+ . Although the structure of Cu-MOF is well known, the detailed structure of its counterpart Fe-MOF is not known in detail.
Figure 3 : Structure of Cu-MOF .
Table 3: Literature data on surface area, pore size and pore volume of sorbent [ [liv] ]
BET surface area, m2/g
BJH pore volume, cm3/g
BJH pore size, nm
1.6 ADSORPTION OF AROMATIC SULFUR COMPOUNDS ON MOFS
MOFs have been used as sorbents for many kinds of adsorbates. However, there is limited number of publications on adsorption of large organic molecules such as aromatic compounds. Recently, adsorption of aromatic sulfur compounds on some MOFs has been investigated [28, [lv] ] as a way to obtain low-sulfur liquid fuels.
1.7 RESEARCH OBJECTIVE
General objective of this research is to investigate adsorption and desorption of aromatic sulfur compounds on certain MOFs. One specific objective of this study is to investigate how adsorption and desorption of aromatic sulfur compounds on MOF depend upon temperature. Another specific objective is to learn about mechanism of adsorption-desorption of aromatic sulfur compounds on MOF.
These goals will be accomplished by using the fallowing approaches:
1) we will determine if any chemical reaction takes place during interaction of aromatic sulfur compounds with MOF;
2) we will determine the dependence of adsorption capacity upon temperature;
3) we will determine the stoichiometry of adsorption complexes formed by molecules of aromatic sulfur compounds with MOFs;
4) we will determine the kind of binding in adsorption complexes using fluorescence spectroscopy.
CHAPTER 2: EXPERIMENTAL
2.1 SULFUR AROMATIC COMPOUDS
Thiophene, BT, DBT and 4,6-DMDBT and n-tetradecane were obtained from Sigma Aldrich and used as received. The MOFs Basolite C300 and F300 were bought Sigma Aldrich, and have been activated prior to adsorption tests.
2.2 MODEL FUELS
Model liquid fuels containing thiophene, BT, DBT, or 4,6-DMDBT were prepared by dissolving respective aromatic sulfur compounds in tetradecane at initial concentration of 0.033M (for thiophene, BT, and DBT), and at 0.022 M (for 4,6-DMDBT ). Tetradecane was used due to it has high boiling point and similarity to aliphatic hydrocarbons found in diesel fuel.
2.3 ACTIVATION OF MOFS BEFORE ADSORPTION
MOFs need to be activated prior to adsorption experiments because MOFs readily adsorb water under ambient conditions. Activation of MOFs includes desorption of water, oxygen and other volatile impurities present in MOF since their synthesis. Activation of Cu-MOF was performed via heating at 150 Â°C for 24 hrs in vacuum of < 1x10-4 Torr as a reported [ [lvi] ].
Figure 4: Apparatus for activation of MOFs
Figure 4 shows our homemade apparatus for activation of MOFs. This setup contains one oil roughing pump and turbo pump. Vacuum gauge controller shows current pressure as progress time can be watched. Up to four glass of quartz vessels with MOFs can be loaded into the activation setup at the same time.
2.4.1 UV/VIS SPECTRA OF AROMATIC SULFUR COMPOUNDS
The UV/VIS spectrometer, model Cary 50 Biorad 50 was applied to measure UV/VIS spectra of solutions of aromatic sulfur compounds.
2.4.2 CALIBRATION CURVE OF AROMATIC SULFUR COMPOUNDS
The UV/VIS spectrometer, model Cary 50 Biorad 50 was also applied to construct a calibration curve and determine the equilibrium concentration of aromatic sulfur compound in solution before or after adsorption.
2.5 ADSORPTION AT CONSTANT TEMPERATURE
Figure 5: Apparatus of adsorption/desorption bounded to shaker
Apparatus for adsorption/desorption has been built by us. Its two main components, shaker and heater, are connected to each other as shown in Figure 5. Thermocouple is connected to the vessel with suspension of model fuel and MOF to "read" temperature at any given time. Four adsorption vessels with suspension can be loaded into the apparatus at the same time.
We used Cu-MOF for adsorption after its outgassing. After outgassing, we mixed 0.30 g Cu-MOF with 50 ml of 0.033M solution of either thiophene, BT, DBT. Alternatively, we used 50 ml 0.022M solution of 4, 6-DMDBT. Our solutions were placed onto the shaker attached to the heater for adsorption-desorption experiment at constant temperature. Adsorption was allowed to proceed for up to 11 hours at room temperature (298 K), under continuous shaking.
Periodically, we collected a small aliquot (ca. 0.2 ml) of suspension, centrifuged it to obtain a clear supernatant. We analyzed the clear supernatant by HPLC-UV and UV-Vis spectroscopy to determine chemical composition of fuel after adsorption, and remaining concentration of aromatic sulfur compounds.
Specifically, at constant temperature, we also determined kinetics of adsorption of DBT with Cu-MOF from 0.033 M, 0.00033 M and 0.000033 M 25 ml solutions by collecting 0.6 ml aliquots 9 times (1, 2, 3, 4, 5, 6, 8, 10 and 24 hours). Moreover, kinetics of adsorption of DBT with Fe-MOF from 0.00033 M 50 ml solutions was determined by collecting 0.3 ml aliquots 9 times (10, 20, 30, 40, 50, 60,75, 90 minutes and 24 hours).
2.6 ADSORPTION AT VARIABLE TEMPERATURE
We used Cu-MOF for adsorption of variable temperature after its outgassing. We mixed 0.30 g Cu-MOF with 50 ml of 0.033M solution of either thiophene, BT, or DBT, or with 50 ml 0.022M solution of 4,6-DMDBT. Our solutions were placed onto the shaker connected to the heater for adsorption-desorption process.
For adsorption at variable temperatures, the setup was additionally equipped with Proportional Integral Derivative (PID) temperature controller from Auber Instruments, Inc. This controller was programmed by us to achieve variable temperature as below, following manufacturer's instructions. At constant temperature, we wait until After 11 hours at 298 K to reach thermodynamic equilibrium, 0.2 ml suspension of model fuels were collected for analysis. The rest of solutions was continued 1 hour for heating up to reach 348 K degree still by stirring. Than 11 hours lasted at 348 K degree, again for equilibrium after collected 0.2 ml suspension, continuingly 1 hour waited to reach 388 K degree and 11 hour to reach thermodynamically equilibrium to 388 K and lastly spontaneously cooled. After succession 11 hours, we collected a small aliquot (ca. 0.2 ml) of suspension, centrifuged it to obtain a clear supernatant. We analyzed the clear supernatant by HPLC-UV and UV-Vis spectroscopy to determine chemical composition of fuel after adsorption, and remaining concentration of aromatic sulfur compounds.
Figure 6: Sequence of heating and sampling for analysis
2.7 CHEMICAL ANALYSIS BY HPLC-UV (HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY)
To determine any molecular products of chemical reaction of aromatic sulfur compounds under conditions of adsorption, we used HPLC instrument model Gold from Beckman Coulter. This instrument is equipped with model 168 UV-Vis detector and C18 5-Î¼m column. Eluent was 25 % vol. water / 75 % vol. acetonitrile. UV-Vis detection was monitored at Î»=326 nm for DBT and 4-MDBT, and at Î»= 299 nm for BT (adsorption maxima). Supernatant solutions were injected directly to the HPLC loop; injection volume is 5 Î¼L; flow rate is 0.8mL/min.
2.8 FLUORESCENCE SPECTRA OF MOFS AND SULFUR AROMATIC COMPOUNDS
Fluorescence spectroscopy had been used to detect interaction between MOFs and aromatic sulfur compounds. This spectrometer is equipped with ??
2.9 QUANTUM MECHANICAL CALCULATIONS OF STRUCTURE OF REPRESENTATIVE AROMATIC SULFUR COMPOUND
In this computational research, we examine the bond length, dipole moment, UV/VIS spectra and the vibrational spectra of Dibenzothiophene using the Gaussian viewâ„¢ and Gaussian program. The theories and methods used for this project are as follows: our methodology will be using the Gaussian â„¢ program to determine the structure of the molecular compound specifically UV/VIS spectra and the vibrational spectra of the molecules. The theories underpinning our analysis are respectively: the Vsepr theory to comprehend to orbital nature of the molecular compounds and Schrodinger's equation for the calculation of the frequencies using various basis sets.
Our experiment was performed using the Gaussian software, which is a software package for use in computational chemistry. Gaussian allows us to calculate electronic structure and spectra of molecules and atoms. We preformed our experiments on a computer running on Linux. We used GaussView 5.0 which is the graphical user interface to the Gaussian program. After building the molecules and cleaning and symmetrizing, we saved them as an input files. We then ran several calculations. For geometry optimization of Dibenzothiophene (DBT) C12H8S, we used Hartree-Fock level of the theory with the standard 6-311** basis set and DFT (B3LYP) level of theory using the standard 6-311G** basis set for IR spectra, TD-DFT (B3LYP) level of theory using the standard 6-311G** basis set for UV/VIS spectra.
CHAPTER 3: RESULTS AND DISCUSSION
3.1 UV/VIS SPECTRA OF AROMATIC SULFUR COMPOUNDS AND UV/VIS CALIBRATION CURVES
Figure 7: UV/VIS Spectra of Benzothiophene in solution of n-C14H30
Peak identification for BT in tetradecane was conducted after using standards, by comparison with spectra reported in the literature [??]. As seen in the UV/VIS Spectra of solution of benzothiophene in Figure 7, BT gives a distinct narrow peak at 299 nm.
Figure 8: UV/VIS Spectra of solution of Dibenzothiophene n-C14H30
Peak identification for solution of DBT in tetradecane was conducted using standards, by comparison with spectra reported in the literature [ [lvii] ]. UV/VIS spectra of Dibenzothiophene in Figure 8 show that BT gives a distinct narrow peak at 327 nm.
Figure 9: UV/VIS spectra of solution of 4,6-Dimethyldibenzothiophene n-C14H30
Peak identification for 4,6-DMDBT in tetradecane was conducted by comparison with spectra in the literature [??]. UV/VIS spectra of 4,6-Dimethyldibenzothiophene in Figure 9 show that 4,6-DMDBT gives a well-defined peak at 327 nm. We note that UV-Vis absorption spectrum of 4,6-DMDBT resembles the one of DBT. This is due to the structural similarity between two molecules: the only difference s the presence of methyl substituent groups in the ring.
Figure 10: Calibration curve for quantitative determination of BT by UV/VIS spectroscopy
We have determined the UV-Vis absorption calibration curve for diluted solution of BT at wavelength Î»=299 nm (at absorption maximum), Figure 10. The graph is essentially linear within the range of concentrations 0 - 1 mM.
For the solution of 4, 6-DMDBT, its UV-Vis calibration curve as measured at Î»=327 nm is shown in Figure 11. The graph is essentially linear within the range of concentrations 0 - 1 mM. For other aromatic sulfur compounds studied (thiophene and DBT), calibration curves are linear (data not shown).
Figure 11: Calibration curve for quantitative determination of 4,6-DMDBT by UV/VIS spectroscopy
3.2 CHEMICAL COMPOSITION OF MODEL FUELS BY HPLC-UV AFTER ADSORPTION
It was determined by HPLC-UV that aromatic sulfur compounds do not react with MOFs during the adsorption desorption experiments. Specifically, HPLC-UV data indicate that there are not any molecular products of chemical reactions between BT (Figure 12) or DBT with MOF in liquid phase during adsorption.
Figure 12: HPLC-UV spectrum of BT onto Cu-MOF at 299 nm
Figure 12 shows the HPLC-UV time-absorbance trace for BT in C14H30 after adsorption/desorption from CuMOF. Major peak at retention time about 4.2 min belongs to BT as shown by injection of BT standard.
Figure 13: HPLC-UV time-absorbance 3D trace for BT in C14H30
Figure 13 shows the 3D HPLC-UV time-wavelength-absorbance trace for BT in C14H30 after adsorption/desorption from Cu-MOF. Major peak at retention time about 4.2 min belongs to BT as shown by injection of BT standard.
Figure 14: HPLC-UV spectrum of DBT onto Cu-MOF at 326 nm
HPLC-UV spectrum of DBT onto Cu-MOF give identical peak at 326 nm with around 14 minute retention time as seen in Figure 14.
Figure 15: HPLC-UV time-absorbance 3D trace for DBT in C14H30
Figure 15 shows the 3D HPLC-UV time-wavelength-absorbance trace for DBT in C14H30 after adsorption/desorption from Cu-MOF. Major peak at retention time about 14 min belongs to DBT as shown by injection of DBT standard. Traces appearing at Î»=180-220 nm originate from instrumental noise.
3D form graphic of DBT was gathered after adsorption/desorption process via sample DBT solution and compare with standard of DBT.
3.3 KINETICS OF ADSORPTION AT CONSTANT TEMPERATURE
The 0.000033 M DBT with Cu-MOF, UV/VIS cannot be used to measure kinetics of adsorption (too low concentration of DBT and too low optical absorbance). For 0.033M solution of DBT with Cu-MOF, adsorption kinetics is more complicated than the first order rate law.
Figure 16: Kinetic of adsorption of DBT on Cu-MOF at constant temperature at 0.033M
We used directly absorbance-adsorption time graphic due to absorbance of Cu-MOF on aromatic sulfur compounds is directly proportional to adsorption concentreation in solution as showed in Figure 10 and 11. Adsorption of Cu-MOF on DBT is completed almost at 3 hours.
3.4 TEMPERATURE-PROGRAMMED ADSORPTION/DESORPTION OF AROMATIC SULFUR COMPOUNDS ON MOF
Adsorption capacity of Cu-MOF C300 versus BT, DBT, thiophene and 4,6-DMDBT in tetradecane has been calculated at 298 K, 348 K, 388 K and after cooling to 298 K. We found the different adsorption capacity of C300 versus aromatic sulfur compounds.
According to adsorption theory, there are 2 types of adsorptions [ [lviii] ]. First is Physical (Physisorption) which the adsorbate molecules are attracted by weak van der Waals forces towards the adsorbent molecules. For this kind adsorption, adsorption occurs via exothermic process, it is more possible at low temperature and if temperature is increased, desorption occurs (Le-Chatelier's principle). Secondly, Chemical (Chemisorption) adsorption is when adsorbate molecules are bound to the adsorbent molecules by chemical bonds. In this type adsorption, bonding between adsorbate and adsorbent on surface area occur via exothermic process.
Figure 17: Extent of sulfur compounds adsorption at equilibrium under 298, 348 and 388 K
Figure 18: Adsorption/desorption cycle of Thiophene
Figure 18 shows that initial concentration of thiophene on Cu-MOF in solution decreases from 0.033 to ?? upon adsorption at 298 K, then increases to ?? upon heating up to 348 K. Lastly, by cooling our solution spontaneously , again desorption occur in solution till up ?? These adsorption values predicts that bonding of thiophene to Cu-MOF occurs via physisorption because at higher temperature, desorption occurs.
Specifically, as it indicated in Figure 17 adsorption capacity of Cu-MOF towards BT as high as 78.23 g S/kg sorbent at 288 K; when temperature increases by 50 K to 348 K, the adsorption capacity of Cu-MOF remains relatively the same at 79.52 g S/kg sorbent. After heating another 50 K, the adsorption capacity drastically drops down to 9.29 g S/kg sorbent Lastly, by cooling our solution spontaneously (for almost 2 hours), adsorption capacity in the solution reaches 63.69 g S/kg sorbent. Those results show that adsorption capacity of Cu-MOF is about the same before and after adsorption, and Cu-MOF can be used as a reversible adsorbent by arranging a correct adsorption-desorption temperature program.
Figure 19: Adsorption/desorption cycle of BT
For DBT as it showed in Figure 17, as high as 74.16 g S/kg sorbent of adsorption capacity of Cu-MOF towards 4,6-DMDBT, when adsorption temperature increases by 50 K degree, adsorption capacity become lower with 62.04 g S/kg sorbent ,again by heating up to 50 K degree, desorption occur capacity down to 37.33 g S/kg sorbent Lastly, by cooling our solution spontaneously (almost 2 hours), again adsorption occur in solution till up 91.7 g S/kg sorbent of initial concentration.
Such data indicate that adsorption of DBT on Cu-MOF occurs with physisorption because at high temperature desorption occurs, so DBT may be used as recycling adsorbent by arranging adsorption-desorption temperature programming.
Figure 20: Adsorption/desorption cycle of DBT
Specifically, as it indicated in Figure 17 adsorption capacity of Cu-MOF towards 4,6-DMDBT as high as 82.83 g S/kg sorbent at 288 K; when temperature increases by 50 K to 348 K, the adsorption capacity of Cu-MOF remains relatively the same at 82.83 g S/kg sorbent. After heating another 50 K, the adsorption capacity still remains to 84.97 g S/kg sorbent. Lastly, by cooling our solution spontaneously (for almost 2 hours), adsorption capacity in the solution reaches 83.1 g S/kg sorbent.
As a result of adsorption-desorption cycle of 4,6-DMDBT tested by us, adsorption of 4,6- DMDBT on Cu-MOF occurs via chemisorption, since there is no change in adsorption capacity over a rather wide temperature range.
Figure 21: Adsorption/desorption cycle of 4,6 DMDBT
For comparison, adsorption capacity of commercially used Y-Zeolite for DBT is around 6 g S/kg sorbent [ [lix] ].
3.5 STOICHIOMETRY OF ADSORPTION COMPLEXES
In our experiments, we assume the existence of stoichiometric adsorption complexes formed by each aromatic sulfur compound with Cu-MOF. We calculated stoichiometric ratios (moles of adsorbed aromatic sulfur compound) / (moles of Cu-MOF present).
Table 4: Stoichiometry ratio of Cu-MOF toward to aromatic sulfur compounds.
Table 4 shows those calculated ratios. Cu-MOF and Thiophene BT, DBT and 4,6 DMDBT was analyzed to see binding ratio at 298,348,388 and after spontaneously cooling.
3.6 RAMAN AND FLUORESCENCE SPECTRA OF ADSORPTION COMPLEXES
Fluorescence spectra of Cu-MOF complexes with BT were measured under excitation at 300 nm that corresponds to absorption maximum of BT at the longest wavelength. This is the excitation from HOMO to LUMO of that molecule i.e. excitation from ground singlet state S0 to the first excited singlet state S1. Therefore, fluorescence excited at 300 nm originates from S1 state to ground singlet state of BT molecule .
Figure 22 : Fluorescence spectra of solid suspension of Cu-MOF in BT
Fluorescence measurement was conducted at the same excitation wavelength 300 nm as excitation of fluorescence for BT in solution. For solid BT, there is very small fluorescence signal at 315 nm. This is consistent with the reported small cross-section of fluorescence for sulfur-containing aromatic heterocyclic compounds [ [lx] ]. There is virtually no fluorescence from Cu-MOF dispersed in solid C19H40, On the other hand, in the solid suspension of BT in Cu-MOF, there is a significantly higher fluorescence signal at 315 nm. Fluorescence peak at 315 in Figure 22 for solid suspension is similar to that observed for BT in solution; therefore, we observe an enhancement of intrinsically weak fluorescence from BT after interaction of BT with Cu-MOF. This enhancement of fluorescence intensity is apparently due to formation of adsorption complex with Cu-MOF and formation of coordination bonds. Thus, our spectroscopic data are consistent with formation of stoichiometric adsorption complex of BT with Cu-MOF as shown independently, in Table 4.
3.7 COMPUTATIONAL RESULTS
Figure 23: Gaussian view of DBT
Table 5: Bond lengths and Angles: Hartree-Fock level of the theory using the standard 6-311** basis set for optimization
Table 6: Bond lengths and Angles: the DFT (B3LYP) level of theory using the standard 3-21G** basis set
Figure 24: IR spectrum of DBT with Gaussian programming
Based on the C-H and C-S stretching, DFT (B3LYP) level of theory using the standard 6-311G** basis set for IR spectrum, we arrive at the values 3214.35 cm-1 and 781.797 cm-1 spectrum.
UV/VIS Spectrum (with Gaussian program calculation)
Figure 25: UV-VIS spectrum of DBT with Gaussian programming
Based on the C-H and C-S stretching TD-DFT (B3LYP) level of theory using the standard 6-311G** basis set for UV/VIS spectrum, we arrive at the values 239.65nm and 232.35nm and 303nm spectrum
As a result; our calculation by TD-DFT (B3LYP) level of theory using the standard 6-311G** basis set for UV/VIS, is consistent with experimental values.
In this project we used Gaussian software to calculate various characteristics of Dibenzothiophene(DBT) C12H8S. We then compared our results to literature and experimental values from assorted research papers. Although we experienced some deviation between our calculated values and the literature and experiment, we can explain these deviations. Overall we feel that we have obtained largely valid values for the characteristics of our molecular compounds - NMR properties, UV/VIS spectra and the vibrational spectra, bond length, dipole moment, frequencies etc. In future experiments we hope to be able to look at the molecular compound with oxidation state compounds, examining the molecule in three dimensions and the grouping the motions of the molecule to achieve closer values to the literature and experimental values.
CHAPTER 4: APPENDICES