The Complex Compound of Amino Acids With Titanium (III) as a Method to Control and Synthesis of Different Structures of TiO2 Nanoparticles; Usage as Photocatalysts to Oxidize Alcohols to Aldehyde


 Different types of the amino acids (Glutamine, Glycine, Alanine) were used to coordinate TiCl3 in order to investigating the best precursor for synthesis of TiO2. Also, a full investigation was carried out to synthesis four different structures of TiO2 nanoparticles [TiO2 (A0.8R0.2), TiO2 (A0.6R0.4), TiO2 (Anatase), and TiO2 (Rutile)]. Oxidation of derivatives alcohol to their corresponding aldehyde through the obtained nanoparticles, as a photocatalyst, under UV light was considered to investigate the best structure of TiO2. Different physical-chemical analyses were applied to investigate the result. The result showed that the titanium dioxide nanoparticle, synthesized from glycine was obtained at the least temperature and was chosen as a precursor to synthesis of four different types of TiO2. All the synthesized TiO2 were applied for oxidation of benzyl alcohols into benzaldehyde, as a test, and TiO2 (A0.6R0.4) could give the best result (87% efficiency). Then it was used to oxidize benzyl alcohol, 4-cholorobenzyl alcohol, 4-nitrobenzyl alcohol and 4-methoxybenzyl alcohol to their corresponding aldehyde and efficiency were 74, 92, 87, and 65% respectively.

Amino acids are natural substances with a low-temperature melting point, soluble in water, and also they have an active side to make complex compounds with titanium ions, so they are active and effective ligands to synthesize complex compounds of titanium. Titanium dioxide obtained from these types of complex compounds can be obtained at low temperature and less time with high purity [17].
Amino acids have different categories, specifying with their side chains, including amino acids with electrically charged side chains, amino acids with polar uncharged side chain, amino acids with hydrophobic side chain, and special case.
On the other hand, titanium dioxide nanoparticle has different phases and structures, including anatase, brookite, and rutile. There is also an artificial structure called Degussa, which is made by 80% anatase and 20% rutile, and it shows a high ability for different application such as photocatalyst. So they illustrate different abilities based on their molecular structure, and according to these properties, synthesizing TiO2 with different ratio of phases can be very essential, especially when they are used as optical and photocatalyst.
TiO2 nanoparticles, as the most important semiconductor photocatalysts, were confirmed as an effective and selective agent in partial oxidation of benzyl alcohols to corresponding benzaldehydes [27][28][29]. It has been confirmed that the structure of TiO2 and the adsorbed oxygen molecules on the surface of TiO2, as two essential factors, have strong influences on the selective physicochemical activity [30][31][32].
In this study, we focused on the photocatalytic application of titanium oxide nanoparticle in the oxidation reaction of benzyl alcohols into aldehydes, as an especial and important reaction in industries. To this aim, we synthesized nanoparticle of titanium oxide [TiO2 with 20%anatase-80% rutile (A0.8R0.2), TiO2 with 60% anatase-40%rutile (A0.6R0.4), TiO2 with 100%anatase (A), and TiO2 with 100%rutile (R)] through the complex compounds of amino acids of Glycine, L-Alanine, and glutamine, which belong to the special case, amino acid with hydrophobic side chain, and amino acids with a polar uncharged side chain respectively.
For comparison purposes, the home-prepared nano-TiO2s have been applied in the photocatalytic oxidation of benzyl alcohols to the corresponding aldehydes in acetonitrile at ambient temperature, under UV irradiation. The partial photocatalytic oxidation of benzyl alcohol, 4-cholorobenzyl alcohol, 4-nitrobenzyl alcohol and 4-methoxybenzyl alcohol to the corresponding aldehydes was investigated with home-prepared nano-TiO2 samples. Also, the influence of the substituent group was studied in the photocatalytic oxidation reaction.

Experiment
Characterizations. XRD pattern was obtained by GNR Explorer advance using 1.541 A(Cu-Ka). Firefly 7.1.G software was used for quantum-chemical modelling of the molecule and complex. Synchronous thermal analysis (simultaneous recording of TG and DSC curves) was performed on a Netzsch STA 449 F3 Jupiter installation in air. BET analysis to measure the surface activity of the nanoparticles was carried out by capillary condensation of nitrogen at 77 K on a micrometric Belsorp-mini II. To estimate the mesoporosity and pore size distribution function, the Barrett-Joyner-Halenda method (BJH) was used. The microscopic images of the nanoparticles were detected by TESCAN BRNO-Mira3 LMU to study the morphology of the nanoparticles, which was accompanied by EDX analysis. TEM images were taken by LEO 912AB through dispersing by ethanol. Estimating the energy bandgap of the nanoparticles was carried out by UV-Vis spectroscopy varian 50. The melting points of the products were determined with an Electrothermal Type 9100 melting point apparatus.
FT-IR spectra were recorded on pressed KBr pellets using an AVATAR 370 FT-IR spectrometer (Therma Nicolet spectrometer, USA) at room temperature in the range between 4000 and 400 cm -1 with a resolution of 4 cm -1 . NMR spectra were recorded on an NMR Bruker Avance spectrometer at 300 MHz in CDCl3. Mass spectra were recorded with a CH7A Varianmat Bremem instrument at 70 eV electron impact ionization, in m/z (rel %).
All yields refer to the isolated products after purification by column chromatography. The photocatalytic oxidation experiments were performed in a capped-sealed test tube equipped with a 100 W Hg lamp (UVA, UVB Light). The system was cooled by water circulating and the temperature was about 20℃.
Material. All the main precursor, including ligands and solvents, were purchased from Merck and Sigma-Aldrich and were used as received without further purification. The purity determinations of the products and the progress of the reactions were accomplished by TLC on silica gel Polygram STL G/UV 254 plates.

Synthesis of complexes
General method. Using 100% natural-renewable amino acids as ligands, which decomposes at <550℃ with high uniformity to synthesize titanium oxide make an advantage of using these types of ligands to use as ligands for making complexes as precursors for obtaining nanoparticle. All the procedure was carried out under a nitrogen atmosphere to avoid any oxidization. Due to using the derivatives of amino acid, as the ligand, the observation was almost the same with a difference in time of different step. Deionized water and methanol with 60 and 40% were used respectively, as solvents to solve the ligands. Then NaOH was added to make the environment of the reaction neutral. After that, TiCl3 was added dropwise to make precipitate. The colour was changed from colourless to green and then it changed to blue. Then it stayed for days at room temperature and after some days the blue colour changed to white suspension. The mentioned method was repeated for Glutamine (L 1 ), Glycine (L 2 ), and L-Alanine (L 3 ).

Synthesizing nanoparticles of TiO2
General method. The obtained complex compounds were used as a precursor to isolate titanium oxide nanoparticle of TiO2. Thermal decomposing was the method used to transfer complex compound to the nanoparticles. To test the proper temperature and time, a small amount of the complex compound was analyzed thermally. The following procedure was applied for the TGA analysis (Thermal Gravimetric Analysis).
For this purpose, each complex compound was placed in a closed aluminium crucible and the temperature was set between 25-900 °C. The heating rate was 10 deg/min. According to the obtained result from the analysis and XRD pattern, the product, from the complex of glycine and TiCl3 (ML 2 ), was obtained at a lower temperature (around 525C), the obtaining titanium oxide nanoparticle from Glutamine+ TiCl3 (ML 1 ) and L-Alanin+TiCl3 (ML 3 ) was resulted at around 860C and 900C respectively. So ML 1 was chosen, as the most proper precursor, for obtaining further products, including TiO2 (A0.8R0.2), TiO2 (A0.6R0.4), TiO2 (A), and TiO2 (R).
The mentioned products were obtained by controlling the temperature (Fig.1). TiO2 (A), as the first phase of TiO2, was obtained at around 525 ℃, TiO2 (A0.6R0.4) at around 650℃, TiO2 (A0.8R0.2) at about 750℃, and finally, TiO2 (R) was obtained at 800℃. Photo-oxidation reaction was carried out in a capped-sealed test tube containing homeprepared nano-TiO2 (3mg) suspended in a solution consisting of 2 ml benzyl alcohol (1mmol) and acetonitrile as solvent. The mixture was stirred using constant magnetic stirring in different periods under UV light irradiation. The progress of the reaction was monitored by TLC (eluent: n-hexane/EtOAc 5:2). After completion of the photocatalytic reaction, the photocatalysts were separated by centrifugation at 10,000 rpm rate. The quantitative value of benzaldehyde was obtained by column chromatography.

Complex compounds
Quantum chemical modelling of the complexes. To reveal the structure and electronic properties of the isolated complexes I to III, we performed a DFT calculation of the molecular structure of both organic molecules: HL 1 , HL 2 , HL 3 , and molecules that model the structure of the isolated complexes: Ti(L 1 )2OH (I), TiL 2 (OH)2 (II), TiL 3 (OH)2 (III). The molecular structure of the calculated organic molecules and complexes is shown in Fig. 2   The disadvantage of the Ti (L 1 )2OH (a2), TiL 2 (OH)2 (b2) and TiL 3 (OH)2 (c2) structures corresponding to the chemical analysis data is the low coordination numbers of titanium: five and four, instead of the expected coordination number six. It can be assumed that, in crystalline form, the titanium atom, being in forms (a2), (b2), and (c2), can complete its coordination sphere due to the interaction with the oxygen atoms of neighbouring complex molecules, forming polymer structures. At the same time, the possibility of completing the coordination sphere of titanium in Ti(L 1 )2OH exists due to the additional interaction with the O3 atom of one of the ligand molecules, as in the Ti(L 1 )2OH (a3) molecule (Fig. 2). It was found that the Ti(L 1 )2OH (a3) form is 60 kJ/mol more stable than the Ti(L 1 )2OH (a2) form.
In the case of TiL 2 (OH)2 and TiL 3 (OH)2 complexes, the possibility of completing the coordination sphere of titanium is possible, for example, in the dimer molecules TiL 2 (OH)2 (b3) and TiL 3 (OH)2 (c3) (Fig. 2). It was found that for these molecules the energy of structures (b3) and (c3) are 134 and 135 kJ/moles lower than the doubled energies of structures (b2) and (c2), respectively. The results obtained for Ti(L 1 )2OH, TiL 2 (OH)2, and TiL 3 (OH)2 indicate higher stability of titanium octahedral complexes in comparison with tetragonal pyramidal or tetrahedral ones. Some calculated bond lengths and bond angles for organic molecules and complexes are shown in Table 1. Table 1. The lengths of some bonds R (Å) calculated by the DFT method and the bond angles ω (deg) in the molecules (The numbering of atoms is given in Fig. 2
The transfer of electron density from ligands to titanium atoms occurs so that the electron density on the O1 and N1 atoms directly bonded to the titanium atom, as a rule, changes little and even increases, and on atoms that are not directly bonded to the titanium atom, decreases.
In this case, the electron density on the O1 atom increases especially: charges on this atom lie in the range from -0.698 to -0.702 in organic molecules and -0.756 to -0.797 in complexes.
Note that the released charge on the O2 atom in the Ti( Initially, the ligands are investigated in the free mode. There are different peaks in the free amino acids' derivatives. However, some of them have been recognized as the main functional groups of the amino acids. There are small differences between -amino acids (Lalanine and glutamine) and -amino acid (Glycine). In both cases, there is a sharp peak at around 2130 cm -1 , which related to N-H stretching in NH 3+ group [43]. The peaks between 1640 and 1610 cm -1 are related to the deformation of NH 3+ . There is another peak related to the ionic carboxyl group (around 1410 cm -1 ). However, this is hard to see compared to other peaks. Also, COOmode is observable near 1600 cm. Furthermore and according to some kinds of literature, ammonium can be seen at 2600 cm −1 [44]. The main difference between glycine, as -amino acid, and -amino acids (L-alanine and glutamine) is a broad carboxyl peak in near 1600cm -1 After coordination of the metal ion, there are some changes, which effected on the vibration energy of some of the mentioned peaks. In the case L 1 +Ti (Glutamine complex) ( Fig.3), both peaks corresponding to the stretching carboxylate group and stretching vibrations of ammonium reduced to around 1632 and 2038 cm -1 from 1638 and 2043 cm -1 respectively. Also, the peak related to the ionic carboxyl group was reduced from 1415 to 1410 cm -1 . Such this change may be justified by η-mode coordination of titanium ion to the resonance forms of the carboxylate group.
In the next case, L 2 +Ti (Glycine complex) (Fig. 4), there is a huge reduction in ammonium peak, and it changed from 2213 to 2136 cm -1 . Also, the carboxylate peak was deformed after complexation. Also, a new peak at around 3440 cm -1 badges of intermolecular bond in O-H stretching.
In the last case, L 3 +Ti (L-Alanine) had its unique effect on the metal ion (Fig. 5). The significant change occurred in the carboxylic acid (O-H stretching), which reduced the peak from the around 2742 to 2729 cm -1 . Also, the peak related to ammonium changed and raised to around 2615 cm -1 from around 2600 cm -1 . Elemental analysis of the complexes was taken in table 3.

Nanoparticles
The obtained nanoparticles were labelled as followings; TiO2 (A), TiO2 (R), TiO2 Anatase phase, as the first phase of TiO2, was obtained at around 525℃ (Fig. 6 c) Where, K is shape factor (about 0.9), λ is the wavelength of the X-ray source (1.5406Å), β is the breadth of the observed diffraction line at its half intensity maximum (400).  FT-IR spectroscopy. Investigation of the surface state of prepared nanoparticle was carried out by FT-IR spectrum, adjusted between 400-4000 cm −1 (Fig. 7).
There is a broad absorption band between 3400 and 3500 cm -1 , which related to the absorption of the H2O molecules' stretch on the surface of the TiO2 samples. It is clear in the cases A0.6R0.4 and R [45]. Besides, some components band gap illustrates various kind of interaction of water molecules with TiO2, containing their absorption into the mesoporous [46]. The broad absorption band between 400 and 800 cm -1 and the maximum at around 1000 cm -1 corresponding to Ti-O stretches in TiO 6 octahedral of different types of distortion. Some adsorptions with very low intensity, around 1300 cm -1 , show the presence of CO2 , which exist in the air and absorbed by the surface of the TiO2 nanoparticles. FESEM and EDX analysis. Because each crystal facet provides especial properties so the morphology of the obtained nanoparticles was investigated by FESEM image (Fig.8). All the nanoparticles were obtained in very high purity, as they are seen in the figures. This is one of the most important properties of the oxide nanoparticles, which synthesized through organometallic complex compounds [47].
There are very uniform in their surface and the shapes of the nanoparticles have been investigated by TEM image. There is no accumulation and tendency, which almost always is due to the high temperature and stability of the nanoparticles. Also, the high porosity on the surface of the samples made them be used as good photocatalysts for radicalism reactions.
The obtained size for all the nanoparticles was calculated; mean size for TiO2 (A0.2R0.8) structure was around 25nm, and for TiO2 (A0.6R0.4), anatase, and rutile was around 20nm, 29nm and 31nm respectively. The results obtained from the FESEM were well equal to what obtained from the XRD analysis. Besides, EDX of the nanoparticles, to investigate the amount of the elements was carried out (Fig. 9). According to the analysis, all the nanoparticles were synthesized with high purity (Table 5).   Anatase phase is illustrated by figure 10 c. Anatase as the first phase of TiO2 was obtained as a truncated bipyramidal shape constructed by eight {101} and two {001}, which is observable by TEM and FESEM image (inserted in figure 10c). According to the databases [48], this type of shape is obtained with hydrated surfaces.
Figure10d provides information about the rutile phase. Rutile generally obtained with no regular shape and this is the most stable phase. However, here, the rutile phase was obtained with truncated bipyramidal shape, and the same as the anatase phase, with hydrate surface.
All the samples' properties have been taken in the figure. Surface analysis. The typical 2 adsorption-desorption isotherms (BET method) and the pore distribution curve of samples showed in Fig. 11. The micropores volume has been calculated in the P/Po range of 0-0.990 to collect information about the genesis of porosity (Fig.11a). BET analysis has been used to investigate the surface activity of the samples (Fig.11b), in addition, to determine pore diameter, volume and distribution BJH method have been carried out (Fig. 11c). Furthermore, Langmuir plot, which provides a mean of determining surface area according to monolayer coverage of the solid surface through the adsorptive, has been considered (Fig. 11d). DeBoer t-plot has also been done to determine the external surface area and micropore volume of microporous materials. This plot is according to standard isotherm and thickness curves that describe the statistical thickness of the of adsorptive on a non-porous reference surface (Fig. 11e). The relationship between the pore shape and the adsorption-desorption isotherm is H3. These materials have slit-shaped pores. Because of the tensile strength effect, which probably happens for N2 at 77K in the pressure between 0.4 and 0.45, the desorption curve of H3 hysteresis contains a slope related to the force on the hysteresis loop. Also, according to the IUPAC classification, they seem to belong to the type III, which they should be characteristic of macroporous adsorbents, having pore sizes exceeding 50.0 nm. However, the result provided much less than 50.0 nm (Table   6), and according to the data, they are characteristic of adsorbents that are mesoporous [49], which classified as IV or V. Classification as model V is more logical due to the appearance schematic and it is more logical than the model IV. Thee is also another study on the classification of the pore size, which recently provided by Mays [50], and according to his accurate investigation, the obtained nanoparticles can be also classified as super-nanopore (10-100nm).
The specific surface activity of the samples has been also determined separately by BET analysis (Table 6).
Adsorption is a phenomenon, which depends on the adsorbate on the adsorbent. Langmuir is one of the methods, which can describe the distribution of metal ion such as titanium between two phases of liquid and solid. According to this method, monolayer adsorption on the surface holding a limit number of adsorption site of identical strategies with no transmigration of adsorbate in the plane surface [51]. After filling the site, adsorption will not happen to the site. It illustrates that the surface has no more volume and it is saturated.
The data obtained from this analysis can be seen in table 6.
The energy bandgap of the samples was calculated by the Tauc plot (2); Where α is Absorption coefficient, hν is the photon energy,α 0 , h is the constants, Eg is the optical bandgap of the material and n is between 0.5 and 3 (it depends on the electronic transition).
The energy band gap is received from the plot made by the curves of (αhν) 2 versus hν.

Why is the nanocomposite-TiO2 (A0.6R0.4) the best?
The process of the photocatalytic reaction is explained in detail in many types of research and our previous work [15,45,52].
First, it should be explained what properties could affect the photocatalytic and oxidation reactions. Everything comes from the physical properties such as surface activity, total pore volume and pore diameter, besides, the molecular structure of the samples can be also regarded as an important parameter. Furthermore, energy bandgap is also very important; less energy bandgap makes less distance for transferring an electron from the valence to conduct the band to make the photocatalytic reaction.
The larger surface activity provides a better condition for absorbing light to react.
According to the BET surface activity, the highest surface activity is related to TiO2 (A0.8R0.2) (30.63 m 2 g -1 ), while the least is related to TiO2 (A0.6R0.4) (11.29 m 2 g -1 ). In another word, although the average pore diameter of the TiO2 (A0.6R0.4) has the largest amount, pore volume and surface activity is almost the smallest, which shows almost the lowest porosity is related to this sample and it means one of the most effective parameters in the photocatalytic reaction has not been very efficient. It is also confirmed by BJH, Langmuir, and DeBoer t-plot analyses. The next parameter is the molecular structure.
The molecular structures of anatase and rutile phases have been already reported (Fig.14).
However, about the two others [TiO2 (A0.8R0.2), TiO2 (A0.6R0.4)], as they are a composition of anatase and rutile phase, the same as Degussa p25, still nobody could find the exact crystalline structure [53], which is due to the lack of enough knowledge and proper methodology for finding the crystalline contents. So due to being both anatase and rutile phases in TiO2 (A0.8R0.2) and TiO2 (A0.6R0.4), we consider them as Degussa p25, as the nearest structure can be purposed so probably the properties will be the same. In both anatase and rutile phases, while the irradiation is occurring O-2p and some of Ti-3d are exist in the valence band, which make a powerful hybridization between the orbitals of p and d from O-2p and Ti-3d respectively, and indeed, they make the bonding state in the valence band region and provide a bed to transfer of photogenerated holes. In the opposite band, conduct band is composed by Ti 3d state, which makes interaction with some O-2p and Ti-3d states. In this band, the main hybridization occurs between Ti-3d and O-2p states, as the antibonding state.
However, in the cases TiO2 (A0.8R0.2) and TiO2 (A0.6R0.4), there is no knowledge about the exact hybridization and standpoints of the atoms and structures. Energy bandgap is another key factor, which has to be considered. According to the Uv-Vis spectroscopy of the samples and Tauc's plot, the smallest energy bandgap is related to TiO2 (A0.6R0.4) with around 3eV, while others were obtained around the same amount of 5eV (Fig.  15). It means the electrons in the valence band of TiO2 (A0.6R0.4) need to take less energy (almost half compare to other samples) to get the conducting band, so more electron have this possibility to get the conducting band to continue the photocatalytic process, in this case, oxidation reaction. And as a result, more holes are created in the valence band. Even the problem, which is too fast recombination in TiO2 when used as photocatalyst [54], will not effective because the process can be continued as fast as it is possible. Also, it should be mentioned that although surface activity and porosity are two important factors in photocatalytic reactions, reducing energy bandgap can be more effective, which is obtained by controlling the condition to obtain a sample with the almost the same phases of anatase and rutile.