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This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1

Photocatalytic selective oxidation of terminal methyl group of dodecane

with molecular oxygen over atomically dispersed Ti in mesoporous SiO2

matrix

Jae Yul Kim, a Ji-Wook Jang,

a Duck Hyun Youn,

a Eun Sun Kim,

a Sun Hee Choi,

b Tae Joo Shin

b and Jae

Sung Leec 5

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX

DOI: 10.1039/b000000x

Extraordinarily high selectivity (80-93%) for oxyfunctionalization of terminal methyl group was discovered in photocatalytic selective oxidation of dodecane with molecular oxygen in a continuous flow system under mild gas phase reaction conditions over mesoporous TiO2-SiO2 mixed oxide photocatalysts. 10

The oxygenated hydrocarbon products were mainly aldehydes with carboxylic acids and ketones as minor products. By dispersing most of Ti atomically in tetrahedral coordination in SiO2 matrix, the oxygenated products were stabilized by diluting contiguous Ti sites present on the surface of TiO2 particles. The preferential oxidation of terminal methyl groups was ascribed to the extensive C-C bond breaking by photogenerated holes prior to oxyfunctionalization. 15

Introduction

The selective oxidation of alkanes to more valuable products is of great importance in chemical and pharmaceutical industry. For instance, aldehydes and carboxylic acids are intermediates for fatty acids via olefin hydroformylation and subsequent 20

oxidation.1-3 In most oxidation reactions by molecular oxygen, the strength of the involved C-H bonds controls the rate and selectivity4-6, because the initial step of alkane activation is the hydrogen abstraction to form a reactive intermediate. Hence, preferential oxidation at specific C-H bonds remains a 25

challenging task. Since the cleavage energy of terminal C-H bonds in n-alkanes is stronger than that of secondary C-H bonds (e.g., 410 vs. 397 kJ/mol for propane), the selectivity for terminal methyl group oxidation remains low, typically below 10% in C10-alkanes.7 30

Attempts have been made to improve the terminal oxidation selectivity through rendering spatial constraints on n-alkanes. Thus, Thomas et al. have demonstrated the oxidation of terminal CH3 and penultimate CH2 of linear n-alkanes (C5-C8) by regioselective catalysts using O2 in a liquid-phase reaction at mild 35

condition (373-403 K).8, 9 Molecular sieves CoAlPO-18 and MnAlPO-18, in which Co and Mn were introduced into the framework of aluminophosphate (AlPO) showed the unprecedented high oxyfunctionalization selectivity (~65.5%) at the terminal CH3 group when conversion was kept low (~7%). In 40

contrast, Modën et al. could not observe any specific regioselectivity in catalytic oxidation of n-hexane and cyclohexane using MnAlPO-5 or MnAlPO-18.7 Their terminal oxidation selectivity of 7-8% was similar to that predicted from relative C-H bond energies in n-hexane. They rationalized the 45

lack of regioselectivity to their large intracrystalline cages. Interesting results were reported for short-contact-time reactors with six orders-of-magnitude smaller residence times than those for comparable industrial liquid-phase processes. Thus, O'Connor and Schmidt used single layers of Pt–10%Rh gauze catalyst to 50

achieve 70% selectivity to oxygenated hydrocarbons (OHCs) at ~20% n-hexane conversion.10 The OHCs contain less than ~30% of aldehydes and the rest of oxygenates was mainly dimethyltetrahydrofuran, indicating that terminal-selective oxidation of alkane by molecular oxygen was difficult in this 55

system too. In a completely different approach, we have discovered that photocatalytic gas phase oxidation of long chain alkanes can produce OHC with mesoporous TiO2-SiO2 mixed oxide photocatalysts, in which Ti are ultimately dispersed in atomic 60

scale under UV light irradiation. In particular, we obtained the surprisingly high terminal selectivity of 80.5-92.9% among formed aldehydes, carboxylic acids and ketones (37-62 % of the ptroducts) at 29.1-47.2% conversion of dodecane. Hence photocatalytic oxidation could be a highly selective process of 65

terminal methyl group functionalization at mild temperature and pressure (80 °C and atmospheric pressure). As we selected dodecane with 12 carbons or 1.4 nm-long straight chain as a substrate, synthesis of mesoporous TiO2-SiO2 mixed oxides was critical with pores big enough for dodecane to penetrate into inner 70

pores freely. We realized the synthesis of the mesoporous TiO2-SiO2 mixed oxides by adopting a relatively smaller tetramethyl ammonium hydroxide solution (TMAOH) molecule compared with the one used in our previous work11 as a structure directing agent. In addition to the extraordinarily high selectivity for 75

oxyfunctionalization of terminal methyl group, the results have a fundamental significance because photocatalytic oxidation of

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View Article OnlineDOI: 10.1039/C3GC41343H


2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

organic compounds has been traditionally used for total oxidation or mineralization for environmental protection rather than selective partial oxidation.

Experimental

Catalyst synthesis 5

Titanium (IV) isopropoxide (TTIP, 97%, Sigma-Aldrich), tetraethyl orthosilicate (TEOS, 98%, Acros Organics), tetramethyl ammonium hydroxide solution (TMAOH, 25 wt% solution in methanol, Sigma-Aldrich), 2-propanol (100%, JT baker) were used without further purification. For given amounts 10

of TTIP and TEOS depending on a desired Ti/Si ratio, 3.33 x (TTIP + TEOS) moles of 2-propanol, 0.035 x (TTIP + TEOS) moles of TMAOH, and 3.33 x (TTIP + TEOS) mole of distilled H2O were dropped into the Teflon autoclave bottle in this order. This transparent solution was hydrolyzed at 170 °C for 15 h 15

followed by filtering with 500 ml of distilled H2O. Powder sample was dried overnight at 100 °C and calcined at 500 °C for 10 h.

Catalyst characterization

The textural properties such as surface area, total pore volume, 20

pore size distribution (PSD) and microporosity of TiO2-SiO2 mixed oxide were determined using N2 physisorption measurements. Prior to measuring an isotherm, the sample was degassed at 250 °C for 3 h. N2 adsorption-desorption isotherms were obtained at -196 oC using a nanoPOROSITY-XQ Analyzer 25

(Miraesi, Korea). The surface areas were calculated by using the BET equation within a relative pressure range (p/p0) of 0.05-0.20. The total pore volume was determined from the amount of nitrogen adsorbed at the highest relative pressure of p/p0 = 0.995. The PSD plots were determined by applying the Barrett-Joyner-30

Halenda (BJH) model to the desorption isotherm. Microporosity was assessed by adopting t-plot method setting 3.5-7.0 Å as a statistical thickness range. HRTEM, EELS and SAED pattern images were taken by HR-STEM-2200FS (JOEL JEM-2200FS with Image Cs-corrector) at 35

the National Center for Nanomaterials Technology in Pohang, Korea. 50 µl of the solution, in which 10 mg of powder sample was dispersed in the 4 ml of absolute ethanol, was dropped on a holey carbon grid for TEM observation. The porosity and crystalline phase of the photocatalyst were determined using both 40

small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques. 2D SAXS images were collected at PLS-II beam line 9A of Pohang Accelerator Laboratory (PAL) in Korea. The incident X-ray was set to 20.065 keV (λ = 0.6202 Å) via LN2-cooled Si (111) double crystal monochromator. 45

Sample-to-detector distance was ~4495 mm and 2D SAXS images were collected by a Rayonix SX165 CCD detector. WAXD data were acquired by X-ray diffractometer (The PANalytical X’Pert PRO MPD X-ray diffraction system) with monochromatic Cu Kα radiation (λ = 1.5408 Å) at 40 kV 50

accelerating voltage and 30 mA emission current. The X-ray absorption fine structure (XAFS) measurements were conducted on PLS-II beam line 7D of Pohang Accelerator Laboratory, Korea. The radiation was monochromatized using a Si(111) double-crystal monochromator and the incident beam 55

was detuned by 15% for harmonic rejection. The spectra for K-

edges of Ti (E0 = 4966 eV) were taken in transmission mode at room temperature. The intensity of incident beam (I0) was measured with a He-filled IC Spec ionization chamber and that of transmitted beam (IT) with a N2-filled chamber. Helium gas was 60

steadily flowed into the sample chamber between I0 and IT where the sample was mounted. And, we only considered dehydrated samples to avoid broadening of the tetrahedral pre-edge peak by hydration of Ti instead of forming Ti-O-Si bond network. The energy was scanned by 5-eV steps over 4766-4916 eV, 1-eV 65

steps over 4916-4956 eV, 0.25-eV steps over 4956-4996 eV, 2.0-eV steps over 4996-5700 eV for 2, 2, 2, 3 sec per point for integration, respectively. The obtained data were analyzed with Athena in the IFEFFIT 1.2.11 suite of software programs.12 The pre-edge background was removed by using a simple linear fit. 70

The resulting spectrum was then normalized by approximating the post-edge background function, evaluating it at the edge energy and dividing the spectrum by the pre-calculated normalization constant. Diffuse Reflectance Infrared Fourier Transform (DRIFT) 75

analysis was performed with Nicolet 6700 FT-IR Spectrometer (912A0654, Thermo Scientific), equipped with a detector at -196 oC in liquid N2 and a DiffuseIR cell (Pike). About 1 spoonful of sample scooped up by a 5 mm round spatula was put on the die with 5 mm diameter and pressed with stainless rod paired with 80

die cell. The spectra were recorded for 128 accumulations at a resolution of 4 cm-1. UV-Vis DRS were taken with UV-2401PC (Shimadzu). Pyrex glass holder filled with powder sample of ~1 mm depth was transferred to the diffuse reflectance die for scanning. BaSO4 was used for reference standard. Autozero 85

setting was conducted at every scan. The ICP analysis was conducted with ICP AES (SPECTRO-VISION).

Photocatalytic partial oxidation of dodecane

Photocatalytic reactions were performed in anhydrous environment with a continuous flow system described in Fig. S1. 90

The temperature of reactor was adiabatically controlled by thermocouple-controller setup. The carrier gas (Ar) flowed through a dodecane saturator immersed in a constant temperature silicon oil bath. A 450 W Hg lamp (7825-34, Ace Glass) was used as a UV-Vis source. Light intensity was 0.81 W/cm2 95

measured by a photometer (ORIEL 70260 with 70282 head). Tubular quartz glass was used as a cooling jacket and a light window, in which cooling water was circulated. Photocatalyst powder immersed in ethanol solution was sprayed over the 24 quartz glass rods on a 300 °C hot plate for coating. Chemical 100

products were analyzed by an on-line gas chromatograph (HP 6890) equipped with a DB-5 column (Agilent Technology, 125-5532) and 2 auto sampling valves for TCD and FID analysis. The products were identified with a GC (HP 6890)-MS (HP 5973) equipped with a DB-5ms column (Agilent Technology, 122-105

5563).

Results and discussion

Textural properties of mesoporous TiO2-SiO2 mixed oxide photocatalysts

Like chemical catalysts in general, textural properties of the 110

photocatalysts are critical for their performance. Adsorption isotherms of N2 synthesized with different Ti/Si ratios (0.04,

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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3

0.11,

Fig. 1 a) Nitrogen sorption isotherms at -196 oC and b) BJH pore size distribution of TiO2-SiO2 mixed oxides with different Ti/Si ratios. Inset in a) highlights the region of hysterisis between adsorption and desorption 5

branches of the isotherms.

0.33, 0.55, and 1.00) were examined in Fig. 1a. The isotherms for Ti/Si=0.04, 0.11 could be classified as the type-II isotherm representing a non-porous or macroporous adsorbent characterized by unrestricted monolayer-multilayer adsorption. 10

But they are different from the typical Type-II isotherm in the presence of the narrow range of plateau above p/p0=0.9, indicating the presence of mesopores. Typical type-IV isotherms were exhibited by samples with higher Ti/Si values with the adsorption plateau above p/p0=0.5. During the desorption process, 15

H2 hysteresis loop was observed indicating the narrow distribution of pore sizes and shapes. It is associated with capillary condensation taking place in mesopores usually formed at interparticle holes and slits. The different natures of isotherms between the two low and three high Ti/Si samples are reflected in 20

pore size distribution in Fig. 1b calculated by the BJH equation. The samples with lower Ti/Si ratios show much broader and larger pores, whereas samples with higher Ti/Si ratios show much sharper distributions in the range of 2-4 nm. The quantitative textural properties are summarized in Table 1. 25

The Ti/Si ratios remained almost identical before and after the synthesis without loss. As Ti content increased, the total pore volume and the average pore diameter of mixed oxides decreased

monotonously. But BET surface area showed a maximum at Ti/Si=0.33. The microporosity of the samples was investigated by 30

Fig. 2 HRTEM images of more than 20 different spots of TiO2-SiO2 mixed oxides; a,b) Ti/Si=0.04, c,d) Ti/Si=0.33. Arrows indicate lattice fringes of anatase TiO2 and insets of a) and c) represent SAED patterns.

the t-plot method as shown in Fig. S2. All data of mixed oxides 35

with different Ti/Si ratios showed negative y-intercepts indicating absence of micropores. It is also supported by the small volume adsorbed at low relative pressures in Fig. 1a in contrast to a typical N2 isotherm of microporous zeolite powders.13 The formation of a single active site photocatalyst by atomic 40

dispersion of Ti atoms in SiO2 matrix is critical for high selectivity to OHC as discussed later. To directly confirm this, HRTEM images of samples with Ti/Si=0.04, 0.11, 0.33, 1.00 were carefully examined as shown in Fig. 2 and S3. Four images of more than 10 different places were pictured for each sample. 45

No TiO2 lattice fringe was detected for samples with Ti/Si=0.04 and 0.11, indicating that most Ti was atomically distributed in these two low Ti/Si samples. From Ti/Si=0.33, lattice fringes of anatase TiO2 cluster of less than 1-2 nm begin to appear as indicated by arrows in Fig. 2d. The TiO2 lattice fringes become 50

more obvious for the sample with higher Ti/Si of 1.00 (Fig. S3). In all cases, SiO2 remained amorphous. Low resolution TEM images of the samples with Ti/Si=0.11 and 0.33 were investigated as the representatives of two groups of the samples with different textural properties. The overall geometry of the sample with 55

Ti/Si=0.11 in Fig. S4a shows deeply developed cracks and irregular surfaces due to the presence of the mesopores. The EELS mapping images show well-distributed Ti, Si and O atoms. The sample with Ti/Si=0.33 shows a relatively large particle with relatively smooth surfaces. More porous structures of the 60

Ti/Si=0.11 sample should have contributed to its larger total pore volume and BJH mean pore diameter than those of Ti/Si=0.33. The SAXS technique is an effective tool to characterize mesoporous solids. As shown in Fig. S5, all SAXS profiles of mixed oxide samples display a broad single peak in contrast to no 65

characteristic peak of TS-1 reference sample without mesopores.


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The origin of scattering peak is attributed to the electron density variation in the sample due to the mesopores. The position of the

Table 1 Textural properties of photocatalysts.

Sample

(Ti/Si)

Actual

Ti/Sia

Total Pore

Volume

/ cm3g-1b

Mean Pore

Diameter

/ nm

BET

/ m2g-1

Pore to

Pore

Distance L

/ nm

0.04 0.043 0.863 5.94 269.6 15.7 0.11 0.118 0.774 4.74 349.1 11.9 0.33 0.293 0.482 3.28 374.9 8.63 0.55 0.587 0.393 3.03 363.5 8.50 1.00 1.071 0.302 2.95 324.8 8.67 TiO2

c - 0.226 - 13.78 -

a By ICP-AES; b By BJH method, c Commercial TiO2 (Junsei).

5

peak maximum, qm, shifts towards a higher wave vector with increasing the Ti/Si ratio. The characteristic length scale (pore-to-pore distance, L) is related to the peak scattering vector qm via Bragg relationship L=2π/qm. The pore-to-pore distance (L) decreased from 15.7 to 8.50 nm with increasing the Ti/Si ratio 10

from 0.04 to 0.55, and slightly increased to 8.67 nm at Ti/Si=1.00. Considering the wall thickness of mesopores, these SAXS results are well consistent with the mean pore diameters acquired from the BJH model. Furthermore, scattering peak became broader with increasing the Ti/Si ratio indicative of the 15

broadened distribution of the pore-to-pore distance. Hence, although the scattering peaks of mixed oxide samples are not as sharp as those of highly ordered mesoporous materials such as MCM-41 or SBA-15,14, 15 the mesoporous structures were indeed formed in TiO2-SiO2 mixed oxide powders. Only single broad 20

peaks of each sample are in line with the poor organization of mesoporous structure with large deviation as confirmed in TEM and PSD results. The introduction of Ti atoms to the silica matrix seems to perturb the pore structure when the TMAOH template is removed during the calcinations. 25

Crystal structure and optical properties of photocatalysts

The crystal structure of TiO2-SiO2 mixed oxide photocatalysts was analyzed by wide angle x-ray diffraction (XRD) in Fig. 3a. The broad peak at 2θ=14-38° originates from amorphous SiO2 peak and a sharper peak at 25.4° present only in samples of 30

Ti/Si=0.55 and 1.00 is due to the (101) plane of anatase TiO2. As more Ti is present in the sample, the diffraction peaks of TiO2

become sharper and narrower, while the intensity of SiO2 decreases. Application of the Scherrer equation to the width of (101) peak gives an average crystal size of TiO2 of 1.6 nm for 35

Ti/Si=1.00. Crystal sizes of TiO2 in other mixed oxides of lower Ti/Si ratios should be less than 1.6 nm. The XRD analysis is not sensitive to smaller particle sizes. The size of TiO2 cluster could also be estimated from UV-Vis DRS of the samples (Fig. 3b). The commercial TiO2 shows an absorption 40

edge of ca. 390 nm or band gap energy of 3.18 eV. This is consistent with the well-known band gap of anatase TiO2 (3.2 eV). The mixed oxides show blue shifts of absorption edge, for example, to 330 nm or 3.76 eV for the sample with Ti/Si=0.04. This blue shift of absorption edge represents the well-documented 45

quantum confinement effect due to small TiO2 particles of less than 3 nm.16-20 Thus even the Ti/Si=1.00 sample containing the largest TiO2 particles (1.6 nm) is within the quantum-size domain

and suffers from the blue shift. The maximum shift observed was about 57 nm, or 0.55 eV for Ti/Si=0.04 sample with respect to 50

X Fig. 3 a) Wide angle x-ray diffraction (WAXD) and b) UV-Vis DRS of TiO2-SiO2 mixed oxide photocatalysts.

bulk TiO2. The size and the band gap energy expansion (∆Eg) are 55

related by the following equation.16

where R=radius of the particle; µ=reduced mass of the exciton, i.e., µ-1= m∗

�

��m

∗�

�� where m∗� and m∗

� are the effective mass of the electron and hole; ε=dielectric constant of the material. Since 60

this equation gives ∆Eg=0.15 eV for R=1.6 nm (Ti/Si=1.00), we can roughly estimate the size of 1.25 nm from ∆Eg=0.55 eV for Ti/Si=0.04. But it is certainly possible that Ti exists also as a single site like TS-1 structure because TS-1 and Ti/Si=0.04 shows the similar blue shift as shown in Fig. 3b. The fact that no 65

detection of TiO2 clusters in the samples with Ti/Si=0.04 and 0.11 in HRTEM images supports this statement. (Fig. 2, S3) To obtain the more specific structural information for these poorly crystalline mixed oxides, we employed XANES analysis to investigate their short range, local structures and probe first-70

shell coordination environment around Ti. Ti(IV) has an empty d0 configuration corresponding to Alg or A1 states in octahedral or tetrahedral symmetry connected to 6 or 4 oxygen anions.21, 22 The first intense single peak in the pre-edge region (4970 eV) in front of main-edge (4987 eV) of TiO2-SiO2 mixed oxide with lower 75


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Ti/Si ratio in Fig. 4 corresponds to excitation of the 1s electron into empty bound states, built from hybridization of 3d and 4p orbitals.

Fig. 4 XANES spectra of TiO2-SiO2 mixed oxides. The bottom spectrum 5

for Ti/Si=0.02 almost coincide with that of Ti/Si=0.04.

This intense peak indicates that the majority of Ti(IV) occupies a tetrahedral symmetry position forming Ti-O-Si bond in silica lattice. From the sample with Ti/Si=0.33, a weak triplet peak is observed at 4974 eV due to Laporte-forbidden A1→T2 transition 10

as in TiO2. This indicates that Ti(IV) of an octahedral symmetry begins to form in the Ti-O-Ti bond network and Ti atoms start to form TiO2 clusters. The small shoulder peaks observable near 4968 eV in the sample with Ti/Si=1.00 indicates an excellent fingerprint for anatase phase formation.23 We also performed 15

quantitative analysis of Ti (IV) in Ti-O-Si and Ti-O-Ti states from pre-edge fitting by linear combination of XANES (Fig. S6, Table S1).11, 23-27 The XANES is the most popular technique for this analysis although other photoelectron spectroscopy techniques using IR and UV also have been reported.28-30 20

Interestingly, the normalized Ti-O-Si concentration for Ti/Si=0.02 was lower than that of Ti/Si=0.04. This was explained by the octahedral “hole” formed by the insertion of Ti(IV) to the only one of the two terminal oxygen units produced from the breakage of the bridged oxygen bond in the original SiO2 25

network.23, 31 The fact that not enough Ti atoms are present to fill this octahedral hole indicates Ti atoms in sample with Ti/Si=0.02 are positioned not in the TiO2 cluster but in the single atomic scale.32, 33 The normalized concentration of Ti-O-Si varied from 0.918 to 0.382 and that of Ti-O-Ti from 0.082 to 0.618, as Ti/Si 30

increased from 0.11 to 1.00. The highest Ti-O-Si fraction was found in Ti/Si=0.04. The fact that 38.2% of Ti(IV) is still located in the tetrahedral symmetry even at the highest Ti/Si ratio of 1.00 suggests that the sizes of TiO2 clusters are still small because Ti-O-Si has to be formed in this case only at boundary of TiO2 35

clusters interfaced with SiO2 matrix. This interpretation goes in line with the XRD, UV-Vis DRS and HRTEM images. From these results we can confirm that the majority of Ti (IV) is distributed atomically across the amorphous SiO2 matrix for mixed oxides with Ti/Si=0.04 and 0.11. 40

Photocatalytic selective oxidation of dodecane

Photocatalytic partial oxidation of dodecane was performed on

these photocatalysts at 80 °C under UV-Vis illumination with a total flow rate of 100 cc/min (285 ppm of dodecane, 11% O2 with Ar balance) for 0.4 g of catalyst coated on quartz rods. We 45

carefully collected and analyzed all the products by GC with

Fig. 5 a) OHCs selectivity vs. dodecane conversion. b) OHCs selectivity with time on stream and c) OHCs yield with time on stream when dodecane conversion was about 29% for each photocatalyst. 50

TCD/FID, and GC-MS, and carbon balance between reacted dodecane and all collected products was more than 90%. As shown in Table S2, 25 OHC products were positively identified by GC-MS.11 The main OHC products were C1-C6 aldehydes that were formed by cracking of carbon backbone of dodecane 55

followed by oxidation of terminal methyl groups. Aldehydes with


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more than 6 carbons were also detected by GC-MS and overall selectivity of aldehydes among all OHC products was as high as 85.3%. Small amounts of carboxylic acids and 2-ketones were

Table 2 Selected numerical data for photocatalytic selective oxidation of dodecane. 5

sample Ti/Si Mass

/ g

Conversion

/ %

OHCs

Sel.

/ %

OHCs

Yield

/ %

Carbon

Balance

/ %

TiO2-SiO2

0.04 0.1225 29.1 61.6 17.9 93.9 0.1957 33.8 59.3 20.0 90.0 0.2735 36.0 56.6 20.4 93.0

TiO2-SiO2

0.11 0.0632 29.4 50.2 14.8 95.4 0.1684 37.6 48.3 18.2 90.3 0.2231 40.7 48.6 19.8 92.2

TiO2-SiO2

0.33 0.0354 29.6 45.5 13.5 93.5 0.0500 40.4 38.5 15.6 92.1 0.0755 42.6 36.7 15.6 92.4

TiO2 ∞ 0.0145 31.6 26.0 8.21 93.2 0.0172 35.4 25.8 9.16 93.2 0.0403 47.2 19.9 9.35 90.9

formed. Only a very small amount of H2 (< 10 ppm) was also detected due to its further oxidation to H2O under this oxidizing reaction condition. In TCD analysis, CO2 was found to be the other main product of the photocatalytic oxidation of dodecane 10

by total oxidation, but CO was not detected at all. Selected reaction data for typical photocatalysts are

summarized in Table 2. Overall, TiO2 with octahedral Ti(IV) or contiguous Ti sites was good at obtaining high dodecane conversions. When these contiguous Ti(IV) sites were diluted by 15

dispersing them in silica matrix, dramatic improvement in OHC selectivity was observed showing the highest selectivity for the photocatalyst with the maximum fraction of isolated Ti(IV) sites (Ti/Si=0.04). Practically important OHC yield (conversion x selectivity) also showed the maximum value for the catalyst. 20

Characteristics of photocatalytic partial oxidation of dodecane are presented in Fig. 5. First, OHCs selectivity was compared at dodecane conversions of 30-45% as shown in Fig. 5a. Since OHCs could be further oxidized to CO2, it is fair to compare the selectivities of different catalysts at the same dodecane 25

conversion. Thus, data in Fig. 5a were obtained by using different amounts of photocatalysts as indicated in Table 2. Similar range of dodecane conversion was obtained with a larger amount of catalyst of lower Ti/Si ratio or a smaller amount of catalyst of

higher Ti/Si ratio. Thus, titanium is the active site of the 30

photocatalytic reaction. As expected, OHCs selectivity decreased as dodecane conversion increased for the same catalyst. TiO2 showed low OHC selectivities below ~25%. The selectivity improves dramatically to >55% by introducing a mixed oxide photocatalyst of Ti/Si=0.04, but further increase in 35

Ti content lowered it to the level of ~40%. Thus, OHCs selectivity varied with Ti/Si ratios; Ti/Si=0.04 > Ti/Si=0.11 > Ti/Si=0.33 > TiO2. This OHC selectivity and yield remained stable with time on stream for more than 20 h as shown in Fig. 5b,c. The highest OHCs yield was obtained for mixed oxide with 40

Ti/Si=0.04, which was 2.2 times larger than that of pure TiO2 (17.9% vs. 8.21%) when conversion of dodecane was ~29%. The highest OHCs yield, 20.4%, was obtained when the highest amount of mixed oxide catalyst was used. Hydrogen was also produced in less than 10 ppm only when mixed oxide was 45

utilized, indicating that mixed oxides tame the oxidation activity of pure TiO2. The detailed product distribution and selectivity of the terminal methyl group oxidation are listed in Table 3. A consistent result was observed in the order of aldehyde selectivity, C2 > C4 > C3 > 50

C5 ≥ C1 ≥ C6 for all TiO2-SiO2 mixed oxides, and C2 > C3 ≈ C4 > C5 ≥ C6 ≥ C1 for pure anatase TiO2. The largest disparity in selectivity between TiO2-SiO2 and pure TiO2 was found in formaldehyde. The selectivity of formaldehyde from mixed oxide samples was more than 3.7 times higher than that of pure TiO2. 55

This appears to be due to the lowest stability of formaldehyde under highly oxidizing reaction condition particularly on anatase TiO2. The overall behavior of the photocatalytic selective oxidation of dodecane could be summarized as follows: (Dodecane 60

conversion: 29-43%, OHCs selectivity: 37-62 %)

The most conspicuous difference between pure anatase TiO2 and TiO2-SiO2 mixed oxides was the overall OHC selectivity against total oxidation to CO2. Under similar dodecane conversions of 65

~30%, OHC selectivity over the mixed oxide of Ti/Si=0.04 was 2.4 times higher than that for pure TiO2. To increase OHCs

Table 3 Product distribution of photocatalytic oxidation of dodecane.a

Sample Ti/Si

Dode.

Conv.

/ %

OHCs

Sel.

/ %

Product Distribution / % Sterm

/ %c C1-ald. C2-ald. C3-ald. C4-ald. C5-ald. C6-ald. C1-C6 ald.b

C1-C6 carbox.

+ ketoneb.

TiO2-SiO2 0.04 29.1 61.6 5.76 18.5 11.5 14.4 6.45 4.38 61.0 (10.9) 39.0 (6.99) 80.5

TiO2-SiO2 0.11 29.4 50.2 6.98 21.2 13.5 16.1 7.37 5.04 70.2 (10.4) 29.8 (4.40) 85.1

TiO2-SiO2 0.33 29.6 45.5 7.36 19.7 16.2 19.2 9.43 6.23 78.1 (10.5) 21.9 (2.95) 89.2

TiO2 ∞ 31.6 26.0 1.55 25.5 20.0 20.2 10.7 7.37 85.3 (7.01) 14.7 (1.21) 92.9

aProduct distribution and selectivity are based on the number of carbon atoms. For example, acetaldehyde selectivity= 2 x moles of acetaldehyde formed/12 x moles of dodecane converted. 70

bYields in parenthesis.


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cTerminal selectivity defined as selectivity of aldehydes (Saldehydes) + selectivity of carboxylic acids (Scarboxylic acid) among all OHCs

selectivity and yield in the series reaction of dodecane partial oxidation (dodecane � OHCs �� CO2), the intermediate OHCs have to be stabilized against total oxidation to CO2. Here 5

we propose that dilution of contiguous Ti sites could give better OHC selectivity by employing TiO2-SiO2 mixed oxides as photocatalysts. When pure TiO2 is used as a photocatalyst, all the carbon atoms of dodecane could contact the surface titanium atoms, so that they are easily oxidized all the way to CO2. If we 10

dilute the active sites by inactive SiO2 by forming single Ti atoms or small TiO2 nano-domains surrounded by SiO2, the high reactivity of TiO2 could be controlled and OHCs’ selectivity could improve. In particular, we were able to synthesize TiO2-SiO2 with most of Ti in atomically dispersed in tetrahedral 15

coordination or very small TiO2 clusters of less than 1 nm in SiO2 matrix. These highly dispersed Ti species allow dodecane to adsorb via a single carbon only of dodecane rather than via many carbons on contiguous Ti sites on TiO2 particles in high Ti/Si mixed oxides or pure TiO2. The latter would lead to useless CO2 20

formation. This dilution of contiguous TiO2 active sites is believed to be the main beneficial effect of utilizing the mixed oxide photocatalysts. Another unique feature of our mixed oxide photocatalysts is their mesoporosity with little micropores. For a large substrate like dodecane, facile transport of dodecane and 25

other products without diffusion limitation is essential for high activity. The most interesting characteristic of the photocatalytic oxidation of dodecane is the extraordinarily high selectivity (80-93%) for oxyfunctionalization of terminal methyl group. We 30

introduced a selectivity term to denote terminal carbon selectivity (Sterm) among all OHC products. Such products from functionalization of the end carbon are aldehydes and carboxylic acids. The rest of the products come mostly from secondary carbon, i.e. 2-ketones. The Sterm was larger than 80% for all the 35

photocatalysts and as high as 92.9% for TiO2. This is the extremely high selectivity values that have never been reported at these high substrate conversions (~30%) in selective oxidation of long chain alkanes by molecular oxygen.8,9 The other main product (2-ketones) is produced from oxyfunctionalization of α 40

carbon next to the terminal methyl group. The high selectivity for terminal methyl group functionalization seems to be the unique property of photocatalytic partial oxidation because it is observed for both pure anatase TiO2 as well as TiO2-SiO2 mixed oxide 45

photocatalysts. Another noticeable feature of the present potocatalytic oxidation is extensive C-C bond cleavage. All the detected products have shorter chain lengths than dodecane, mostly less than C6. This may result from the high reactivity of photogenerated holes produced by light absorption of Ti species: 50

(SiO)3Ti + hν → h+ + e-

The holes can initiate the reaction by breaking C-C bonds of the alkane. In chemical catalysis where the C-C bond cleavage is not extensive, the reaction is initiated by activation of C-H bonds of the starting substrate. Then, weaker secondary or tertiary C-H 55

bonds are more easily activated than that of the strong primary carbon, hence oxyfunctionalization of the terminal methyl group remains minimal.4-7,34, 35 In photocatalytic oxidation, hydrocarbon

fragments formed from the C-C bond scission are already activated species at the terminal carbons, where subsequent 60

oxyfunctionalization takes place readily to form terminally oxidized products like aldehyde and carboxylic acids. The higher Sterm value of TiO2 relative to mixed oxides is also consistent with its higher activity of C-C bond scission. In a related system, it is worthwhile to examine the product distribution of the selective 65

oxidation of n-hexane over TS-2 using hydrogen peroxide.36 There were no OHC products with carbons less than six, indicating absence of C-C bond cleavage. In this mild oxidation, the products were made of 2-, 3-hexanols and 2-, 3-hexanone produced from oxyfunctionalization of secondary and tertiary 70

carbons with complete absence of 1–hexanol or 1-hexanal, i.e. no terminally oxidized products. In a similar vanadium complex/H2O2 system, n-octane produced only secondary alcohols and ketones of the same carbon length (C8).

37 But branched octanes produced some primary alcohols of the 75

shortened carbon lengths by C-C bond cleavage and functionalization of the terminal carbon. The examples show correlation between the C-C bond cleavage activity and selectivity for oxyfunctionalization of terminal methyl group in selective oxidation of alkanes. 80

Conclusions

A new reaction of photocatalytic selective oxidation of dodecane to OHCs has been studied under UV light over atomically dispersed Ti in mesoporous TiO2-SiO2 mixed oxide photocatalysts. The OHCs were mainly composed of aldehydes 85

and carboxylic acids produced by C-C bond cleavage and oxyfunctionalization of terminal methyl groups. Thus, among the total OHC selectivity of 45-62% (the balance is CO2), 61-85% were aldehydes, while carboxylic acids and ketones were 15-39%. Among the formed OHC, the terminal carbon selectivity 90

(Sterm) of aldehydes and carboxylic acids was 80-93%, which represents unprecedentedly high values in selective oxidation of n-alkanes. The high selectivity for terminal methyl group functionalization seems to be the unique property of photocatalytic partial oxidation, where prevalent C-C bond 95

cleavage by photogenerated holes on Ti sites drives preferential oxyfunctionalization of terminal methyl groups. Near atomic dispersion of Ti in SiO2 matrix and unique mesopore structure seems to contribute to enhanced activity and OHC selectivity of mixed oxides with low Ti/Si ratios. 100

Acknowledgements

We thank for H. J. Park for TEM measurements. This work was supported by Basic Science Research Program (No. 2012-017247), and Korean Centre for Artificial Photosynthesis (NRF-2012M1A2A2671779), funded by the Ministry of Science, ICT 105

and Future Planning of Korea.

Notes and references

a Department of Chemical Engineering, Pohang University of Science and

Technology, 77 Cheongam-ro, Nam-Gu, Pohang, Gyungbuk, 790-784,

Republic of Korea. 110


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b Pohang Accelerator Laboratory, Pohang University of Science and

Technology, 77 Cheongam-ro, Nam-Gu, Pohang, Gyungbuk, 790-784,

Republic of Korea. c School of Nono-Bioscience and Chemical Engineering, Ulsan National

Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-5

798, Republic of Korea. Fax: (+82)-52-217-1019; Tel: (+82)-52-217-

2544; E-mail: [email protected]

† Electronic Supplementary Information (ESI) available: [Figures of continuous flow system, t-plot, HRTEM, TEM, SAXS images and linear combination fitting of XANES, results of XANES fitting and product 10

retention time]. See DOI: 10.1039/b000000x/

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TOC graphic

Extraordinarily high selectivity (80-93%) for oxyfunctionalization of terminal methyl group was discovered in photocatalytic oxidation of dodecane over mesoporous Ti/Si mixed oxide photocatalysts. 5


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photocatalytic selective oxidation of the terminal methyl group of dodecane with molecular oxygen...

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