Accepted Manuscript

Mesostructured TUD-C Supported Molybdena Doped Titania as High SelectiveOxidative Catalyst for Olefins Epoxidation at Ambient Condition

Yee Khai Ooi, Leny Yuliati, Djoko Hartanto, Hadi Nur, Siew Ling Lee

PII: S1387-1811(16)00026-3

DOI: 10.1016/j.micromeso.2016.01.016

Reference: MICMAT 7528

To appear in: Microporous and Mesoporous Materials

Received Date: 29 September 2015

Revised Date: 15 December 2015

Accepted Date: 8 January 2016

Please cite this article as: Y.K. Ooi, L. Yuliati, D. Hartanto, H. Nur, S.L. Lee, MesostructuredTUD-C Supported Molybdena Doped Titania as High Selective Oxidative Catalyst for OlefinsEpoxidation at Ambient Condition, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.01.016.

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TUD-C supported Mo-TiO2 oxidative catalyst

Si

O

Al-O

Si

O

Si

O

Al-O

Si

O O

H+ H+

O O O O O O O O O O O O

Brönsted acidity

Lewis acidity

Mo-TiO2

Zeolite crystal

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Mesostructured TUD-C Supported Molybdena Doped Titania as High Selective

Oxidative Catalyst for Olefins Epoxidation at Ambient Condition

Yee Khai Ooia, Leny Yuliatib, Djoko Hartantoc, Hadi Nurb, Siew Ling Leeb*

aDepartment of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310

Johor Bahru, Johor, Malaysia.

bCenter for Sustainable Nanomaterials, Ibnu Sina Institute for Scientific and Industrial

Research, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia.

cDepartment of Chemistry, Institut Teknologi Sepuluh Nopember, Jl. Arief Rahman

Hakim, 60111 Surabaya, Indonesia.

Abstract

Mesostructured Technische Universiteit Delft-Crystalline (TUD-C), a hierarchical

zeolitic material with MFI framework was used for the first time as catalyst support for

the olefins epoxidation. While TUD-C and molybdena doped titania showed considerably

low catalytic activities, the molybdena doped titania supported on TUD-C exhibited

significantly improved catalytic activities with 100% selectivity. Both properties and

catalytic performance of the materials were greatly affected by the molar ratio of Si/Al in

the TUD-C. It has been demonstrated that Mo-TiO2 supported on TUD-C with Si/Al =

10 was an excellent oxidative catalyst for epoxidation of various types of olefins.

Kinetics study revealed that the catalytic reactions followed the first order model.

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Keywords: TUD-C; Epoxidation; Titania; Molybdena; Mesostructure

_______

* Corresponding author. Tel.: +607-5536039; Fax: +607-5536080

E-mail address: sllee@ibnusina.utm.my (S. L. Lee)

URL: http://www.ibnusina.utm.my/catalysis (S. L. Lee)

1. Introduction

Epoxides have been the pivotal intermediates and precursors used industrially for

numerous chemical applications such as fine chemical production, polymer synthesis, and

pharmaceutical utilizations [1-4]. Since respective epoxides are playing crucial parts in

multiple applications, the industrial requirements have been imperative in the

manufacture of high yields and high selectivity towards epoxides production with

moderate condition which is environmentally benign. Nevertheless, the underway

methods employed by the industry are incapable to accomplish satisfactory outcomes due

to low yield and selectivity. Furthermore, usage of dangerous solvent and oxidant in

harsh circumstances has been a major shortcoming for the production of epoxides in

industrial scale [5]. Thus, many researchers have accounted the utilization of transition

metal oxides-based homogeneous catalysts for epoxidation [6-9]. Among the transition

metal oxides reported, molybdena is the most amply used for epoxidation of olefins [7-9].

Despite the effectiveness of homogeneous catalyst, the handling liabilities of

arduous separation remain present. Hence, the usage of heterogeneous catalysts for

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epoxidation, especially titania has drawn intensive attention of the researchers. It was

reported that Mo-TiO2 was a prospective oxidative catalyst [8,9]. However, the low

surface area of the material due to agglomeration has restrained the efficiency of the

catalyst. Therefore, employment of catalyst support for improved dispersion of active

sites was suggested. Amongst, titanosilicate (TS-1) [10-12] supported with bulky surface

area mesoporous silica supports such as the Mobil Composition of Matters (MCM) and

Santa Barbara Amorphous (SBA) have been comprehensively reported [13-15].

Unfortunately, catalytic activity and selectivity of these materials are still considerably

low. Mesoporous zeolite with assorted frameworks has been proposed as likely solution

for ameliorating the problem. Due to its high acidity and high surface area, zeolite has

been utilized as catalyst in multiple catalytic applications. However, the small pore size

of zeolite has caused pore blockage, hence restricting its usefulness in dealing with bulky

substances [16-18].

The aforementioned disadvantages can be resolved by fine-tuning the porosity of

the zeolite. Besides, the fine-tuned mesoporosity could be one of the crucial properties

for the optimal diffusion [19]. The Modernite Framework Inverted (MFI) structured of

Zeolite Socony Mobil-5 (ZSM-5) seemed to be the most eligible catalyst support for

olefins epoxidation [18, 20]. Numerous investigations have been focused on integration

of ordered mesopores into zeolite system. Technische Universiteit Delft-Crystalline

(TUD-C) is a crystallined mesoporous zeolitic material which possesses the constitution

of MFI framework structure zeolite integrated into the mesoporous silica. Most

importantly, pore size of the TUD-C could be easily fine-tuned via hydrothermal

treatment [21]. In fact, TUD-C is the further one-step modification originated from

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Technische Universiteit Delft-1 (TUD-1), thus it can be synthesized via a simple route

with the addition of alumina source. It was reported previously that an aluminosilicate

with hierarchical micro/mesoporous structure, which is characteristically similar to TUD-

1 was acquired under conventional steam-assisted synthesis. The product exhibited

superior catalytic performance compared to both amorphous aluminosilicate and

orthodox ZSM-5 in catalysis concerning bulky reactants [22]. As reviewed by Telalović

et al., the three-dimensional sponge-akin mesoporous substance TUD-1 was forthright to

concoct with high versatility. It could easily be incorporated with metals, divulging

numerous diverse catalytic reactions. Via introduction of bimetallic-TUD-1 catalyst,

synergistic effect between Lewis and Brönsted acid sites could be integrated.

Additionally, efficacious utilizations in redox-, acid- and photo-catalysis have proven

TUD-1 to be an exceptional supporting matter for catalysts, imparting innovative

applications [23]. Nevertheless, utilization of TUD-C as catalyst support remains very

limited and many of its characteristics are still unknown. Therefore, further investigation

on its application is necessary.

Recently, our research group reported the usage of TUD-1 as the support for Cr

doped titania photocatalysts in photodegradation of dyes [13,15]. The improved

photocatalytic activity indicated that TUD-1 is a promising catalyst support. In this work,

feasibility of TUD-C as support of molybdena doped titania catalyst for epoxidation

reactions was explored. Molar ratio of Si/Al in the TUD-C was varied in order investigate

its effect towards physical-chemical properties of the oxidative catalysts. The catalytic

performance of these materials in epoxidation of various olefins was presented.

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2. Experimental

2.1 Catalysts preparation

Molybdena doped titania was prepared by sol-gel method according to the

preceding report [15].The mixture was prepared by mixing titanium tetraisopropoxide,

(TTIP, > 99%), ethanol (EtOH, > 99%) as solvent and acetylacetone (acac, 97%) as

chelating agent according to molar ratio 1 Ti{O(CH3)2} 4 : 100 C2H5OH : 2 C5H8O2.All

chemicals were analytical grade and were purchased from Sigma-Aldrich. Meanwhile,

ammonium molybdate was used as precursor of molybdena and it was dissolved in

ethanol. The molybdena dopant (1 mol%) was added drop-wise into the prepared mixture.

The solution was stirred for 2 h. Solvent evaporation was carried out at 353 K and dried

overnight at 383 K, subsequently calcined at 823 K to obtain 1Mo-TiO2.

TUD-C was prepared through a homogeneous synthesis solution consisting of

water, triethanolamine (TEA, 97 wt%), tetraethylammonium hydroxide (TEAOH, 2 M in

water), tetraethyl orthosilicate (TEOS, 98 wt%), and aluminium isopropoxide (Al(iPro)3,

97%). The molar composition was 1 TEOS: x Al2O3: 0.5 TEA: 0.1 TEAOH: 0.3 NaOH:

11 H2O. A series of TUD-C(x) samples (x = Si/Al molar ratio ranging 10 – 50) were

synthesized by varying the amount of Al(iPro)3 precursor. The solution was allotted to

evaporate at ambient conditions for 24 h to incur a solid gel. Later, the solid gel was

crushed to obtain fine powder. The fine powdered samples were then transferred into an

autoclave and underwent hydrothermal treatment at 403 K for 10 h. The products were

dried at 373 K, and were subsequently calcined in air at 873 K for 6 h to remove the

organic contents.

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For the synthesis of 1Mo-TiO2/TUD-C(x) with different Si/Al molar ratios, 1 mol%

Mo-TiO2 synthesized previously was added into the solution of TUD-C with fixed Si/Ti

molar ratio = 30 before the evaporation procedure. The synthesized samples were

denoted as 1Mo-TiO2/TUD-C(x) with x as the Si/Al molar ratio ranging from 10 to 50.

For comparison purpose, TUD-C(10) and 1Mo-TiO2 were also synthesized.

2.2 Characterization

All the synthesized 1 wt% molybdena doped titania supported on TUD-C samples

were characterized via powder X-ray diffraction (XRD) analysis for phase purity and

crystallinity determination. The XRD analysis was carried out on a powder Bruker

Advance D8 diffractometer (40 kV, 40 mA) equipped with incident beam CuKa (k =

0.154 nm) monochromator. The step size was 0.0175o with the counting time per step of

8 s. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iS10

spectrometer using an Attenuated Total Reflectance (ATR) accessory, equipped with a

diamond-crystal cell. Nitrogen adsorption-desorption isotherms, surface area and pore

volume of the samples were measured at 77 K via Quantachrome Surface Autosorb-6B

sorption analyzer. Barret-Joyner-Halenda (BJH) model was applied in mesopores

determination. The regular pre-treatment was 523 K for 16 h. Ammonia temperature-

programmed desorption (NH3-TPD) was measured using a Micromeritics TPD 2900 in

the temperature ranging 300 – 1073 K after three sequential saturation procedures using

pure ammonia at 473 K. The acidity formed within the synthesized samples was

examined via FTIR approach using pyridine as a probing molecule. In the FTIR spectra

measurement, a self-supported wafer was engaged within an in-situ stainless steel IR cell

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with CaF2 windows. Prior to the analysis, the samples were heated at 573 K in vacuum

conditions for 1 h to remove the probable moisture and organic impurities within the

samples surface. Subsequently, 10 Torr of pyridine was adsorbed onto the activated

samples at 423 K for 15 min, followed by outgassing at 573 K. The IR spectra of the

samples were recorded at ambient condition in the pyridine vibration region at 1400 –

1600 cm-1 using Shimadzu 8300 FTIR Spectrometer. Transmission electron microscopy

(TEM) images were attained with a JEOL JEM-2011 electron microscope operated at 200

kV and equipped with a Gatan 794 CCD camera.

2.3 Catalytic activity measurement

Olefin epoxidations were conducted in a sample glass vial with sealant at

atmospheric pressure and room temperature 298 K. Three types of analytical olefins

wereselected as reactant which was obtained from Merck, including 1-octene, 99%

(aliphatic), cyclohexene, 99% (cyclic) and styrene, 99% (aromatic). For each catalytic

reaction, olefin (10 mmol) was mixed with 5 mL of acetonitrile as solvent and 50 mg

catalyst. Oxidant H2O2 (30 mmol) was added drop-wise. The suspension was stirred for

24 h at 2000 rpm and subsequently centrifuged and filtered to acquire the product. The

product obtained was analyzed using gas chromatography Shimadzu GC-2014 with FID

detector. The conversion of olefins and selectivity of products respectively were

calculated using the following equations:

���%� =[olefins]������� − [olefins]�������������

[olefins]�������× 100%

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� �%� =peakareaofepoxides

totalpeakareaofallproductsformed× 100%

Where XS is the conversion of olefins (%), Sx is the selectivity of x product (%) in which

x is epoxides formed via oxidation of different olefins. Turnover number (TON) and

turnover frequency (TOF) of the best catalyst were calculated via equations below:

TON =moleofproduct

moleofactivesites

TOF = TON

Reactionduration

3. Results and discussion

3.1 Physical properties and characterization

TUD-C with Si/Al = 10 (TUC-C(10) is white powered sample while 1Mo-TiO2 is

grayish. After supporting on TUD-C, all the samples of 1Mo-TiO2/TUD-C(x) were white

powdery solids regardless the Si/Al molar ratio of TUD-C. Fig 1 shows the XRD patterns

of 1Mo-TiO2, TUD-C(10) and those 1Mo-TiO2 supported on TUD-C samples. As

observed, 1Mo-TiO2 crystallined in anatase phase [JCPDS-84-1286]. Meanwhile, TUD-

C(10) exhibited XRD pattern equated to MFI crystal structure which corresponded well

with that of commercially available ZSM-5 zeolite [22-25]. The two characteristic peaks

at low angle indexed at (101) and (200) appeared as fundamental indication for the

formation of MFI zeolitic framework incorporated in the amorphous silica. As expected,

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all of the 1Mo-TiO2/TUD-C(x) samples showed both the typical peak of the anatase

phase at 2θ = 25.5o and the peaks of TUD-C, strongly indicating successfully loading of

Mo-TiO2 into TUD-C.

It was noticeable that the crystallinity of these TUD-C supported materials

decreased with increasing Si/Al ratio due to the possible increment of amorphous silicate

in the materials. In the other words, the lower Al content resulted in decreased formation

of zeolite crystals. This finding contradicted with previous report which claimed a higher

Al content is unfavorable for the zeolite crystallization [26]. Thus, it is believed that the

loading of Mo-TiO2 during formation of TUD-C has influenced the crystallization of the

zeolitic material. As a result, crystallinity of these TUD-C supported samples increased

with increase of Al content where 1Mo-TiO2/TUD-C(10) possessed the highest

crystallinity. However, further investigation is required to understand the phenomenon.

Via calculations using Scherrer’s equation, it was accounted that the crystallite

size of TUD-C(10) increased from 35 to 82 nm for 1Mo-TiO2/TUD-C(10) sample (Table

1). The increment in crystallite size could be attributed to the successful incorporation of

1Mo-TiO2 into TUD-C framework. As shown in Table 1, crystalline size of the TUD-C

supported samples increased with increasing of Si/Al ratio of the TUD-C in the samples.

The observation could be explicated via the possible particle growth with the presence of

excess silicate [22]. It has been widely accepted that high crystallinity facilitates high

diffusivity of reactants, while low crystallite size is desired for higher surface area,

contributing to the enhanced catalytic activity [24]. Therefore, the current findings

suggested that the optimum Si/Al ratio for TUD-C is 10 as the sample 1Mo-TiO2/TUD-

C(10) has the highest crystallinity and the smallest crystallite size.

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Fig. 2 illustrates FTIR spectra of 1Mo-TiO2, TUD-C(10) and 1Mo-TiO2/TUD-

C(x). Sample 1Mo-TiO2 exhibited a broad peak at regions 450 – 800 cm-1 due to

overlapping peaks of Ti-O-Ti and Ti-OH which are located at 760 and 580 cm-1,

respectively. The bands at around 3400 and 1633 cm-1 are tallied to OH groups.

Meanwhile, TUD-C(10) showed typical peaks of zeolitic ZSM-5 phase at 450 cm-1 (T-O

bend), 550 cm-1 (MFI phase skeletal vibration), 790 cm-1 (Si-O-Si external symmetric

stretch) and 1100 cm-1 (Si-O-Si internal symmetric stretch). The band at 1225 cm-1 which

was assigned to external linkages (between TO4 tetrahedral) appeared as an additional

proof for the existence of ZSM-5 zeolitic functional groups [28]. After introduction of

Mo-TiO2, the intensities of the zeolitic peaks decreased without any significant peak

shifting, implying perturbation of the silica network due to the possible Ti-O-Si and Mo-

O-Si interactions. It was previously reported that the Ti-O-Si and Mo-O-Si bonds

formation could be evidenced by the decrease in peak intensity and peak shifting at 965

cm-1 which is associated with Si-OH bending variations [29]. As shown in Fig. 2, the

band at 965 cm-1 was almost overlapped by the broad band of 1100 cm-1 and it appeared

as a weak shoulder. Therefore, any changing at this band was not detectable.

N2 adsorption-desorption isotherms of 1Mo-TiO2, TUD-C(10) and 1Mo-

TiO2/xTUD-C (x = 10 – 50) are shown in Fig. 3. At P/P0 ranges 0.05 to 0.45, all the

samples exhibited a near lineal uptake of the isotherm, signifying the existence of

diminutive mesopores edging micropores in these samples. It was conceived that TEAOH

utilized in the synthesis procedure has performed as a structure directing agent since it

functioned as the template for formation of micropore in MFI type zeolites at 403 K in an

autoclave [24]. Furthermore, it has also acted as the scaffolding material for the

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formation of meso-structure during aging process at room temperature. At higher range

of P/P0 > 0.45, 1Mo-TiO2 exhibited very narrow hysteresis loop. On the other hand,

TUD-C and all the TUD-C supported 1Mo-TiO2 samples exhibited a step wise shifting of

the isotherm uptake between 0.45 <P/P0 < 0.85, a type IV isotherms with a H1 hysteresis

loop, which is a good indication for the presence of mesopores [24]. As illustrated by the

corresponding BJH pore size distribution plots (Fig. 4), TUD-C and 1Mo-TiO2/10TUD-

C(x) samples displayed a sharp and narrow pore size distribution with an average

mesopore size of 8.1 nm. In contrast, the unsupported 1Mo-TiO2 showed a wide pore size

distribution ranging 10 – 30 nm.

As depicted in Table 1, 1Mo-TiO2 has the lowest surface area and pore volume of

162 m2/g and 0.04 cm3/g, respectively. Meanwhile, TUD-C(10) possessed the highest

surface area and pore volume of 1451 m2/g and 0.73 cm3/g, respectively. As can be seen,

both surface area and the pore volume of TUD-C decreased significantly after

introduction of Mo-TiO2, implying the dispersion of a portion of Mo-TiO2 molecules in

the pores of the TUD-C. These observations were in complete agreement with the XRD

results, confirming the change from well-ordered structure of TUD-C to reduced

crystallinity upon presence of Mo-TiO2 (Fig. 1). As compared to the unsupported 1Mo-

TiO2, sample 1Mo-TiO2/TUD-C(10) has higher surface area of 1034 m2/g, which is 6.4

fold higher than that of 1Mo-TiO2. Conversely, the pore size diameter increased upon

Mo-TiO2 loading into TUD-C. The increment could be attributed to the accumulation of

Mo-TiO2 along the pore mouth and hence caused in rise of the pore width.

On the other hand, the surface area and mesopore volume of the TUD-C

supported samples gradually decreased with the increasing of Si/Al ratio of the TUD-C

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(Table 1). It is note-worthy that the pore diameter of the samples increased linearly from

3.31 to 6.33 nm with the varying of Si/Al ratio from 10 to 50. The phenomenon could be

explained by the formation of more amorphous silicate which induces the particle growth

and structure deformation, leading to lesser surface area and pore volume as increasing of

the pore diameter. In addition, it was believed that at lower Al concentration, the amount

of zeolite precursors left in solution was too little to perform as a scaffolding agent.

Subsequently, larger crystal was formed [30] as evidenced by the growing of crystallite

size of the samples (Table 1). Similar observation was reported in the synthesis of ZSM-5

where the increase of Si/Al ratio resulted in increase of crystal size and mesoporosity [30-

33].

Fig. 5 demonstrates spectra of the temperature-programmed desorption of

ammonia (NH3-TPD) for 1Mo-TiO2, TUD-C(10), and 1Mo-TiO2/TUD-C(x) with the

diverse Si/Al ratios. As can be observed in Fig. 5, sample 1Mo-TiO2 showed a broad

desorption curve at 346 K which was attributed to reduction of molybdenum, Mo6+ + e

→Mo5+ [34]. With the utilization of TUD-C(10) support, the sample 1Mo-TiO2/TUD-

C(10) exhibited another broad peak at 932 K which is commonly assigned to the

desorption of ammonia from the Brönsted acid sites [34]. A small desorption peak at 735

K attributed to Lewis acidity was also detected in this sample. The observation was in

good agreement with previous report which claimed that TUD-C possessed both

Brönsted and Lewis acid sites, where alumina acted as the proton donor (Brönsted acid)

and Si4+ as the electron acceptor (Lewis acid) [34]. Obviously, all the TUD-C supported

samples have higher amount of Brönsted acidity than that of 1Mo-TiO2. However, peak

intensities of both Brönsted and Lewis acids decreased with increasing of Si/Al ratios in

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the TUD-C. The results demonstrated that the sample 1Mo-TiO2/TUD-C(10) has the

highest amount of Brönsted and Lewis acidities as compared to the other TUD-C

supported samples. It was observed that the peak intensity of 1Mo-TiO2/TUD-C(10) was

comparable to that of TUD-C(10), suggesting acidic sites of these samples were kindred.

Although the inclusive quantity of NH3 desorbed from 1Mo-TiO2/TUD-C(10) was lesser

than that of TUD-C(10), 1Mo-TiO2/TUD-C(10) still sustained a high NH3 desorption rate

even at temperatures exceeding 623 K, signifying the immense acidic strength of 1Mo-

TiO2/TUD-C(10). The results showed that the quantity of NH3 desorbed from 1Mo-

TiO2/TUD-C(10) was expressively higher than other 1Mo-TiO2/TUD-C(x) samples of

higher Si/Al molar ratio, indicating higher acid sites concentration in 1Mo-TiO2/TUD-

C(10) [23].

The type of acidity in 1Mo-TiO2/TUD-C(x) catalysts was further examined via

FTIR via pyridine as a probing molecule. Fig. 6 illustrates the FTIR spectra of the

catalysts in the pyridine vibration region after pyridine adsorption and evacuation at 423

K for 15 min. The spectra exhibited adsorption peaks at 1445 and 1545 cm-1 which are

corresponded to Lewis and Brönsted acid sites, respectively [35]. The quantity of Lewis

and Brönsted acid sites in the catalysts was calculated based on the peak area at ca. 1445

and 1545 cm-1, respectively. As shown in Table 2, TUD-C(10) possessed the highest

amount of Brönsted acid sites, followed by 1Mo-TiO2/TUD-C(10). This could be

explained by the presence of aluminosilicate/zeolitic species within the framework of

TUD-C support, thus contributing to the formation of higher Brönsted acidity in this

sample. The amount of Brönsted acidity decreased with increasing of Si/Al molar ratio in

the samples. This is in good agreement with the NH3-TPD analysis where lower Si/Al

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molar ratio contributed to the higher Brönsted acidity (Fig. 5). It is believed that the

competition between 1Mo-TiO2 and unbounded Al2O3 has constrained the formation of

Brönsted acid sites, hence 1Mo-TiO2/TUD-C(50) has the least amount of Brönsted acid

sites. As can be observed, the amount of Lewis acidity in these TUD-C supported

samples also conforms to a proportional decremented trend with the increment of Si/Al

ratio. Among the samples, 1Mo-TiO2/TUD-C(10) exhibited the highest quantity of Lewis

acid sites (Table 2). The phenomenon suggested the possible reaction between the Mo

species and tetrahedral Ti species or directly with Si-O-Si on the SiO2 matrixes, leading

to more formation of Lewis acidic sites in the sample. The statement was supported by

the results of DRUV–Vis analysis, which designated the existence of more tetrahedrally

coordinated Ti species in 1Mo-TiO2/TUD-C(10).

Transmission electron microscopy (TEM) images of TUD-C(10) and 1Mo-

TiO2/TUD-C(10) are depicted in Fig.7. Fig. 7(a) – 7(c) are corresponded to the local

structures of TUD-C(10). Crystallite size for TUD-C(10) was approximately 28 nm (Fig.

7(a)) and the d-lattice spacing was 1.106 nm for peak (101) (Fig. 7(b)) and 0.385 nm for

peak (501) in Fig. 7(c). After introduction of 1Mo-TiO2, the crystallite size for 1Mo-

TiO2/TUD-C(10) increased remarkably to 84 nm (Fig. 7(d)), a good indication for

successful loading of 1Mo-TiO2 into TUD-C. The d-lattice spacing for peak (110) is

0.195 nm corresponded to molybdena (Fig. 7(e)) and the d-lattice spacing for peak

(101)is 0.370 nm attributed to TiO2 (Fig. 7(f)). The measured crystallite sizes are quite

consistent with those calculated using Scherrer’s equation (Table 1).

The elemental compositions of the synthesized samples are tabulated in Table 3.

Both the theoretical and actual Si/Al molar ratios for each catalyst were determined. As

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can be seen, the actual Si/Al molar ratio is slightly lower than that of the theoretical

values. The results are not surprising as it is known that the Si/Al molar ratio in the TUD-

C could be changed after crystallization process. The elemental mapping of 1Mo-

TiO2/TUD-C(10) is shown in Fig. 8. It was observed that all the elements including Mo,

O, Ti, Si and Al present in the sample with a homogeneous distribution. Fig. 9 illustrates

the possible structure of 1Mo-TiO2supported on TUD-C where portion of 1Mo-TiO2

molecules were believed to be loaded into the mesoporous silicate cavity as evidenced by

the decrease of pore volume and increase of pore diameter after loading of 1Mo-TiO2into

TUD-C (Table 1).

3.2 Catalytic testing

Catalytic testing using epoxidation of various olefins as model reaction has been carried

out. Three types of olefin including 1-octene (aliphatic), cyclohexene (cyclic) and styrene

(aromatic) have been chosen as reactants. Tables 4(a), 4(b) and 4(c) list the product yield,

conversion and selectivity percentage for the respective olefins. For the epoxidation of 1-

octene (Table 4(a)), TUD-C(10) catalyzed reaction only yielded 1.07 mmol 1,2-

epoxyoctane with conversion of 10.7%. Meanwhile, negligible amount of 1,2-

epoxyoctane was produced when 1Mo-TiO2 was used as catalyst in the reaction.

Significant increment of 12.8 fold of yield with 27% conversion and 100% selectivity

towards 1,2-epoxyoctane was detected by using 1Mo-TiO2/TUD-C(10) catalyst. However,

both product yield and conversion value decreased when TUD-C of higher Si/Al ratio

was used as support. Similar observation was found for epoxidation of cyclohexene

(Table 4(b)) and styrene (Table 4(c)). As shown in Table 4(b), a total of 4.8 mmol of 1,2-

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epoxycyclohexane with 48% conversion and 100% selectivity was obtained using 1Mo-

TiO2/TUD-C(10) catalyst. The rise in product yield was 11-fold as compared to the

unsupported 1Mo-TiO2. Meanwhile, in the epoxidation of styrene, 6.2 mmol of styrene

oxide (increment of 8 fold) with 62% conversion and 100% selectivity was detected. In

all the epoxidation reactions using different olefins, both the product yield and

conversion value decreased when the Si/Al ratio of the synthesized samples increased.

This phenomenon could be due to less available active sites for reaction, resulting in

lowering of catalytic activity. Besides, usage of TUD-C support has increased the

selectivity from ∼77 to 100% in all the epoxidation reactions. The 100% selectivity of

TUD-C and TUD-C supported materials could be attributed to the mesopores with

narrow pore size distribution in the materials. On the other hand, it was discovered by

Paolo et al. that hydroxylated aluminium oxides were highly selective heterogeneous

catalysts for the epoxidation of numerous olefins. The suggested catalytic sequence

comprised the development of exterior hydroperoxide groups (Al-OOH) by the reaction

of exterior hydroxyl groups (Al-OH) with H2O2 [33]. In the current work, aluminium

oxides might have existed as unbounded alumina and Al species within the

aluminosilicate framework in all the 1Mo-TiO2/TUD-C(x) samples. These Al species

were expected to play an important role in the olefins epoxidation after reacting with

H2O2.

The overall catalytic performance of 1Mo-TiO2, TUD-C and all the TUD-C

supported 1Mo-TiO2 samples in epoxidation of different types of olefins is depicted in

Fig. 10. Amongst, sample 1Mo-TiO2 was the poorest catalyst for olefins oxidation. It had

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been reported that molybdena doped titania acted as oxidative catalyst under the presence

of hydrogen peroxide [36]. The reaction between the transition metals and hydrogen

peroxide formed hydroxyl radical which was essential for the epoxidation of the olefins

through dissociation of peroxo-metal species [36]. Meanwhile, the catalytic ability of

TUD-C was associated to its Brönsted acidity as zeolite and zeolitic materials have been

widely reported as good oxidative catalysts [37]. The current results demonstrated that

1Mo-TiO2/TUD-C(10), 1Mo-TiO2/TUD-C(20) and 1Mo-TiO2/TUD-C(30) samples were

better oxidative catalysts for the olefins epoxidation compared to 1Mo-TiO2 and TUD-C.

The best catalyst for epoxidation of all three types of olefins is the 1Mo-TiO2/TUD-C(10).

The excellent catalytic activity of these TUD-C supported Mo-TiO2 materials was

attributed to synergy effect of 1Mo-TiO2 and TUD-C. It was believed that mesopores in

TUD-C has reduced the zeolite confinement effect [38, 39]. As a result, the bulky

reactants could easily access the active sites available at molybdena doped titania through

mesoporous zeolite cavities, without causing pore blockage. Besides, the high surface

area of the TUD-C supported materials might have facilitated a better dispersion of the

active sites on the support. It also aided the adsorption of the reactant molecules at the

active sites, leading to enhanced catalytic activity. In addition, the high crystallinity has

contributed to the high diffusivities of materials. As observed, the catalytic activity of the

materials was proportional to the crystallinity. The co-existence of Brönsted and Lewis

acid sites appeared as another key factor for the improved catalytic performance of Mo-

TiO2/TUD-C materials. According to Next Nearest Neighbour Theory [40], an optimized

ratio between the two acidities is crucial in order to achieve good catalytic performance.

The material would be catalytically inactive if it possesses only Brönsted or Lewis acidity

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[41]. The relatively weak activity of 1Mo-TiO2/TUD-C(40) and 1Mo-TiO2/TUD-C(50)

would be due to their low acidity, surface area and crystallinity as Si/Al ratio increased.

Among the three olefins used, the 1Mo-TiO2/TUD-C(10) appeared to be best

suited for epoxidation for styrene (6.2 mmol of yield), followed by cyclohexene (4.8

mmol of yield), and the least suitable for the epoxidation of 1-octene (2.7 mmol of yield).

Table 5 lists the turnover number (TON) and turnover frequency (TOF) of 1Mo-

TiO2/TUD-C(10) in the epoxidation of various olefins. The epoxidation of styrene

recorded the highest TON and TOF of 99.03 and 4.13 h-1, respectively. The high catalytic

activity of the sample in styrene expoxidation could be explained by the existence of

electron donating/withdrawing group in the reactants and also the stability of the

carbonium ions produced. After reacting with the Brönsted acid sites at TUD-C, styrene

was expected to form the most stable carbonium ion because of the presence of electron

donator of phenolic group. The delocalized electron was donated to the neighbouring

carbon atom due to conjugation effect. Meanwhile, the cyclohexene generated the second

most stable carbonium ion due to existence of two methylene groups which acted as

electron donators. The reactant 1-octene formed the least stable carbonium ion attributed

to the terminal double bond which possesses only single alkyl group [42,43]. Besides,

molybdena doped titania was believed to react with hydrogen peroxide to generate the

peroxo-species which has contributed to hydroxyl radical formation [44]. The reaction

between hydroxyl radical and the carbonium ion produced the epoxides as end product.

Further investigations are required to confirm the mechanism of these novel 1Mo-

TiO2/TUD-C oxidative catalysts.

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Figure 11 shows the concentration of products versus time using 1Mo-TiO2/TUD-

C(10) catalyst. As observed, the epoxides produced followed the descending trend of

styrene oxide > 1,2-epoxycyclohexane > 1,2-epoxyoctane. At the first 5 hours of reaction,

all the olefins formed epoxides rapidly. Rate of the reaction slowed down after 5 hours,

heading towards the equilibrium plateau. It may suggest that the reactions were

approaching the optimum yield production with the completion of the epoxidation. The

order of reaction was determined for epoxidation of various olefins (Fig. 12). All the

epoxidation reactions established similar straight line plots with regression value, R2

close to 1in the graph ln[Reactant] against time, indicating the reactions were the first

order kinetic reaction that solely dependent on the reactants only. Amongst, styrene

showed the highest rate constant, k value of 0.1332 h-1, followed by cyclohexane (0.0867

h-1) and 1-octene (0.0390 h-1). A similar kinetics behavior was reported in the literature

when metal oxide was used as catalyst for the olefins oxidation [45].

4. Conclusion

An excellent oxidative catalyst of mesostructured molydena doped titania (Mo-

TiO2) supported on Techniche Universiteit Delft-Cyrstallite (TUD-C) was successfully

synthesized. The results showed that the catalytic activity of Mo-TiO2 in epoxidation

remarkably improved after being loaded into TUD-C. The factors including high surface

area, mesoporosity and acidity were crucial for the enhanced activity. Narrow distribution

of mesopores of TUD-C and TUD-C supported materials contributed to 100% selectivity

towards the epoxides. Among the synthesized materials, sample 1Mo-TiO2/TUD-C with

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Si/Al = 10 was the most efficient oxidative catalyst for epoxidation of three types of

olefins at ambient conditions. The reactivity of the catalyst towards various olefins

followed the descending order of styrene > cyclohexene > 1-octene.The kinetics results

demonstrated the olefin epoxidations followed first order kinetics with a reaction rate of

0.1332 h-1. The research findings strongly suggested that TUD-C, a zeolitic material was

a promising support for Mo-TiO2oxidative catalyst.

Acknowledgement

The authors thank the Ministry of Higher Education, Malaysia (MOHE) and

Universiti Teknologi Malaysia (UTM) for the financial supports through Fundamental

Research Grant Scheme, FRGS (Vote No. R.J130000.7809.4F527) and Research

University Grant (Vote No. Q.J130000.2609.10J66). Yee Khai Ooi acknowledges the

financial support from UTM through UTM Zamalah Scholarship.

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Figure captions Fig. 1. XRD patterns of 1Mo-TiO2, TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Fig. 2. FTIR spectra of 1Mo-TiO2, TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Fig. 3. Adsorption isotherms of 1Mo-TiO2, TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Fig. 4. Pore size distribution of 1Mo-TiO2, TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Fig. 5. NH3-TPD curves of samples TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Fig. 6. Pyridine absorbed FTIR spectra of TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Fig. 7.TEM images of samples TUD-C(a-c) and 1Mo-TiO2/TUD-C(d-e) Fig. 8. EDX elemental mapping of the Mo-TiO2/TUD-C(10) Fig. 9. Schematic diagram showing possible location of Brönsted and Lewis acid sites and the mesoporous zeolite framework of 1Mo-TiO2/TUD-C(10). Fig. 10.Yield of epoxidation of olefins using 1Mo-TiO2 and TUD-C supported 1Mo-TiO2 catalysts. Fig.11. Product yield against time in epoxidation of various olefins using 1Mo-TiO2/TUD-C(10) catalyst. Fig. 12. The kinetic plots of epoxidation of various olefins using 1Mo-TiO2/TUD-C(10) catalyst.

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Table 1 Crystallite size, surface area, pore volume and pore diameter of 1Mo-TiO2, TUD-C(10) and TUD-C supported 1Mo-TiO2 samples. Sample Crystallite

size (nm) Surface area

(m2/g) Pore volume

(cm3/g) Pore diameter

(nm)

1Mo-TiO2 22 162 0.04 9.82 TUD-C(10) 35 1451 0.73 2.96 1Mo-TiO2/TUD-C(10) 82 1034 0.64 3.31 1Mo-TiO2/TUD-C(20) 97 1024 0.61 3.56 1Mo-TiO2/TUD-C(30) 118 832 0.58 4.70 1Mo-TiO2/TUD-C(40) 124 729 0.55 5.84 1Mo-TiO2/TUD-C(50) 135 669 0.52 6.33 Table 2 The acidity amount in TUD-C supported 1Mo-TiO2 samples. Sample Acidity (µmol g-1)

Lewis Brönsted TUD-C(10) 18.1 13.3 1Mo-TiO2/TUD-C(10) 17.3 12.9 1Mo-TiO2/TUD-C(20) 15.9 11.3 1Mo-TiO2/TUD-C(30) 13.6 10.4 1Mo-TiO2/TUD-C(40) 10.8 8.1 1Mo-TiO2/TUD-C(50) 6.9 2.6 Table 3 The EDX elemental analysis of TUD-C supported 1Mo-TiO2 samples.

Sample Chemical composition of Si/Al molar ratio Theoretical Actual

Mo-TiO2/TUD-C(10) 10 9.4 Mo-TiO2/TUD-C(20) 20 17.8 Mo-TiO2/TUD-C(30) 30 28.4 Mo-TiO2/TUD-C(40) 40 36.8 Mo-TiO2/TUD-C(50) 50 46.4

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Table 4(a) Product yield, conversion and selectivity of epoxidation of 1-octene using 1Mo-TiO2 and TUD-C supported 1Mo-TiO2 catalysts. Catalyst 1,2-epoxyoctane

(mmol) Conversion

(%) Selectivity towards

1,2-epoxyoctane (%) 1Mo-TiO2 0.21 12.0 75 TUD-C(10) 1.07 10.7 100 1Mo-TiO2/TUD-C(10) 2.70 27.0 100 1Mo-TiO2/TUD-C(20) 2.16 21.6 100 1Mo-TiO2/TUD-C(30) 1.62 16.2 100 1Mo-TiO2/TUD-C(40) 1.08 10.8 100 1Mo-TiO2/TUD-C(50) 0.54 5.4 100 Table 4(b) Product yield, conversion and selectivity of epoxidation of cyclohexene using 1Mo-TiO2 and TUD-C supported 1Mo-TiO2 catalysts. Catalyst 1,2-

epoxycyclohexane (mmol)

Conversion (%)

Selectivity towards 1,2-epoxycyclohexane

(%) 1Mo-TiO2 0.44 15.4 78 TUD-C(10) 2.33 23.3 100 1Mo-TiO2/TUD-C(10) 4.80 48.0 100 1Mo-TiO2/TUD-C(20) 3.84 38.4 100 1Mo-TiO2/TUD-C(30) 2.88 28.8 100 1Mo-TiO2/TUD-C(40) 1.92 19.2 100 1Mo-TiO2/TUD-C(50) 0.96 9.6 100 Table 4(c) Product yield, conversion and selectivity of epoxidation of styrene using 1Mo-TiO2 and TUD-C supported 1Mo-TiO2 catalysts. Catalyst Styrene oxide

(mmol) Conversion

(%) Selectivity towards styrene oxide (%)

1Mo-TiO2 0.75 16.2 79 TUD-C(10) 3.52 35.2 100 1Mo-TiO2/TUD-C(10) 6.20 62.0 100 1Mo-TiO2/TUD-C(20) 4.96 49.6 100 1Mo-TiO2/TUD-C(30) 3.72 37.2 100 1Mo-TiO2/TUD-C(40) 2.48 24.8 100 1Mo-TiO2/TUD-C(50) 1.24 12.4 100

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Table 5 TON and TOF of 1Mo-TiO2/TUD-C(10) catalyst in epoxidation of various olefins. Olefin TON TOF / h-1 1-octene 43.13 1.80 Cyclohexene 76.67 3.19 Styrene 99.03 4.13

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Fig. 1

Inte

nsi

ty /

a.u

.

1Mo-TiO2/TUD-C(50)

1Mo-TiO2/TUD-C(40)

1Mo-TiO2/TUD-C(30)

1Mo-TiO2/TUD-C(20)

1Mo-TiO2/TUD-C(10)

TUD-C(10)

1Mo-TiO2

80 60 40 20 2-theta / degree

(101)

(200)

Anatase (JCPDS 84-1286

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Fig. 2

Abs

orb

ance

/ a.

u.

Wavenumber/ cm-1

1Mo-TiO2/TUD-C(50)

1Mo-TiO2/TUD-C(40)

1Mo-TiO2/TUD-C(30)

1Mo-TiO2/TUD-C(20)

1Mo-TiO2/TUD-C(10)

TUD-C(10)

1Mo-TiO2

3000 4000 2000 1000

965

1225 1100

790

550 450

1633 3440

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Fig. 3

Ad

sorb

ed V

olu

me

/ cm

3 g-1

0.8

0.6

0.4

0.2

0.2 0.4 0.6 0.8 0.0 1.0 P/P0

1Mo-TiO2/TUD-C(50)

1Mo-TiO2/TUD-C(40)

1Mo-TiO2/TUD-C(30)

1Mo-TiO2/TUD-C(20)

1Mo-TiO2/TUD-C(10)

TUD-C(10)

1Mo-TiO2

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Fig. 4

1Mo-TiO2/TUD-C(50)

1Mo-TiO2/TUD-C(40)

1Mo-TiO2/TUD-C(30)

1Mo-TiO2/TUD-C(20)

1Mo-TiO2/TUD-C(10)

TUD-C(10)

1Mo-TiO2

Ads

orb

ed V

olum

e /

cm3 g

-1

Pore diameter (nm)

5

4

3

2

10 40 20 30

1

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Fig. 5

Am

ou

nt o

f d

eso

rbed

NH 3 /

a.u

.

Mo6+ + e → Mo5+

Brönsted acidity Lewis acidity

1Mo-TiO2/TUD-C(40)

1Mo-TiO2/TUD-C(30)

1Mo-TiO2/TUD-C(20)

1Mo-TiO2/TUD-C(10)

1Mo-TiO2

1Mo-TiO2/TUD-C(50)

TUD-C(10)

Temperature / K 300 600 700 800 400 500 900 1000

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Fig. 6

Ab

sorb

ance

/ a

.u.

1550 Wavenumber / cm-1

1545 1445

1Mo-TiO2/TUD-C(50)

1Mo-TiO2/TUD-C(40)

1Mo-TiO2/TUD-C(30)

1Mo-TiO2/TUD-C(20)

1Mo-TiO2/TUD-C(10)

TUD-C(10)

1450 1500

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Fig. 7

1.106 nm (101)

0.385 nm (501)

0.195 nm (110) 0.370 nm

(101)

28 nm 31 nm

24 nm

84 nm

(a) (c) (b)

(d) (e) (f)

20 nm 10 nm 5 nm

5 nm 10 nm 20 nm

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Fig. 8

Mo

Al

Ti Si

O

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Si

O

Al-O

Si

O

Si

O

Al-O

Si

O O

H+ H+

O O O O O O O O O O O O

Lewis acidity

Brönsted acidity

1Mo-TiO2 nanoparticles

Zeolite crystal

TUD-C mesoporous zeolitic material

Mesoporous silicate cavity

Amorphous silicate

Fig. 9

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Fig. 10

0

1

2

3

4

5

6

7E

poxi

des

/ mm

ol

1,2-epoxyoctane

1,2-epoxycyclohexane

styrene oxide

1Mo-TiO2/ TUD-C(50)

1Mo-TiO2/ TUD-C(40)

1Mo-TiO2/ TUD-C(30)

1Mo-TiO2/ TUD-C(20)

1Mo-TiO2/ TUD-C(10)

1Mo-TiO2 TUD-C(10)

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Fig. 11

Time/ h

5

6 4 2 0 8

3

1

7

Pro

du

ct y

ield

/ m

mol

Styrene oxide

1,2-epoxycyclohexane

1,2-epoxyoctane

6

4

2

0

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Fig. 12

Time/ h

R2 = 0.9975, k = 0.0390 h-1

R2 = 0.9979, k = 0.0867 h-1

R2 = 0.9970, k = 0.1332 h-1

Styrene

Cyclohexene

1-octene

ln [

Rea

ctan

t]

0 2 4 6 8

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Research highlights

• Mesostructured TUD-C supported Mo-TiO2 material is an excellent oxidative catalyst at

ambient condition.

• The catalyst possesses high surface area and mesopores with narrow pore size distribution.

• All TUD-C and TUD-C supported Mo-TiO2 materials exhibited 100% selectivity in

epoxidation of various olefins.