Microporous and Mesoporous Materials 93 (2006) 1–11 www.elsevier.com/locate/micromeso

Synthesis, characterisation and catalytic performance of boron substituted SBA-15 molecular sieves I. Eswaramoorthi, A.K. Dalai

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Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK, Canada S7N 5C9 Received 5 October 2005; received in revised form 16 December 2005; accepted 25 January 2006 Available online 13 March 2006

Abstract Boron substituted mesoporous SBA-15 molecular sieves with varying boron content were synthesised by hydrothermal method. The effects of incorporation of boron in the framework on the structure, crystal parameters, framework vibrations and textural properties of SBA-15 were discussed based on the results from XRD, FT-IR and nitrogen adsorption studies. The morphology changes due to boron incorporation were monitored by SEM analysis. The coordination environment of boron in the framework was revealed by 11B MAS NMR and MQMAS NMR techniques. The type of acid sites created by boron incorporation was analysed by DRIFT study of pyridine adsorbed catalysts. All the characterisation results indicate the successful incorporation of boron in the silica framework of SBA-15 in both tetra and tri coordination. Isopropylation of naphthalene with isopropyl alcohol was carried out to test the acidity of the synthesised materials. The effects of reaction temperature, feed ratio and feed space velocity on naphthalene conversion and products selectivity were discussed. The higher activity in naphthalene conversion and selectivity of di-isopropylated naphthalenes were accounted in terms of the increased acid sites due to boron incorporation in the framework. Ó 2006 Elsevier Inc. All rights reserved. Keywords: SBA-15; Boron;

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B MAS NMR; Naphthalene; Isopropylation

1. Introduction Since the discovery of silica based and metal-substituted mesoporous materials M41S (MCM-41, MCM-48, MCM50), an extensive research progress has been made in the synthesis and characterisation of ordered mesoporous materials due to their high surface area, large and uniform pore size distribution and potential applications in the field of catalysis, separation and adsorption [1–4]. Among these materials, SBA-15, which is polymer-templated silica with hexagonally ordered mesopores, has attracted much attention recently. Its popularity is due to the larger pore size, thicker pore walls and higher hydrothermal stability compared to the well-known MCM-41, which is surfactant-

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Corresponding author. Tel.: +1 306 966 4771; fax: +1 306 966 4777. E-mail address: [email protected] (A.K. Dalai).

1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.01.018

templated ordered mesoporous material [5]. Further, the polymer template employed to obtain SBA-15, poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), is biodegradable and cheaper than the surfactants used in the synthesis of MCM-41. Another attractive feature of SBA-15 is the existence of micropores interconnecting hexagonally ordered mesopores, which make it more suitable for catalysis because these interconnections facilitate diffusion inside the entire porous structure [6]. Reports are available on the usage of SBA-15 as a rigid template for the synthesis of mesoporous inorganic oxides and mesoporous carbons because its inverse replicas retain 3D structures after silica removal due to the existing micropores [7–9]. The improved hydrothermal and thermal stability of SBA-15 make them some of the most promising catalytic materials. However, the materials consisting of pure silica frameworks are of limited use for various catalytic applications because of the lack of acid sites and

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I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

ion-exchange capacity. Generally, incorporation of heteroatom such as Al, Ti, Cu and V within the silica framework of mesoporous materials such as MCM-41 and MCM-48 has been implemented in order to create catalytically active sites, ion-exchange capacity and hence their catalytic activity [10–12]. However, it is very difficult to introduce the metal ions in the silica framework of SBA-15 directly due to the facile dissociation of metal–O–Si bonds under strong acidic hydrothermal conditions. The isomorphous substitution of heteroatom in all silica mesoporous SBA-15 could lead to a useful catalyst for the reactions involving bulkier molecules. Thus, number of direct synthesis and postsynthesis attempts have been made to incorporate Al, V, Zr and Ti in SBA-15 framework [13–17]. Because of the strong acidic media involved during the synthesis, the postsynthesis route found to be more effective over direct synthesis. However, such an approach has been found to reduce the textural properties of the mesoporous framework as well as irregularly distributed active sites. Yue et al. [13] reported the direct synthesis of Al-SBA-15 and found that catalytic activity in cumene cracking is higher as compared to Al-MCM-41. However, the resulting materials have many extraframework aluminium species. The ionexchange, catalytic and adsorptive properties of aluminosilicate molecular sieves originate from acidic sites which arises from the presence of accessible hydroxyl groups associated with tetrahedral aluminum in the silica matrix. The difficulties for the direct synthesis of Al-substituted mesoporous materials under acidic conditions are due to the too easy dissociation of Al–O–Si bond under acidic hydrothermal condition and the remarkable difference between the hydrolysis rates of silicon and aluminium alkoxides. It can be observed that all the heteroatom incorporated SBA-15 showed enhanced activity in various chemical reactions compared to siliceous SBA-15 due to the acid sites created by metal incorporation. Among the various metallosilicates studied, synthesis of borosilicates is considered as important one because they could be useful in synthesising other metallosilicate analogues, which are sometimes difficult to synthesize by direct hydrothermal method [18,19]. Many efforts have been made to incorporate boron in the framework of zeolites as well as mesoporous materials such as MCM-41 [20–22]. Oberhagemann et al. [23] first hydrothermally synthesised highly ordered B-MCM-41 by using different synthesis procedures and found that regularity of the MCM-41 structure depends mainly on the synthesis conditions such as type of base, molarity of amphiphilic molecules, silica source and thermal treatment. Further, they reported from 11B MAS NMR studies that the as-synthesised samples show the presence of boron in tetrahedral coordination, whereas in calcined samples, fraction of tetrahedral boron is converted to trigonal and extraframework species [24,25]. Trong On et al. [26] synthesised B-MCM41 hydrothermally and found that adding boron to the synthesis gel increases the long-range ordering of the materials compared to that of the pure silica analogue. Similar to

MCM-41, incorporation of boron in SBA-15 framework is expected to create acid sites in the neutral framework, which can be used for various chemical reactions. So far no attempt is made to incorporate boron in the siliceous SBA-15 framework, either by direct synthesis or by postsynthesis method. Hence, in this paper, the direct synthesis of B-SBA-15 and their detailed characterisation related to coordination environment of boron in the framework, crystalline nature, textural properties and acid sites are reported. Isopropylation of naphthalene with isopropyl alcohol was carried out as test reaction. The isopropylation of naphthalene is an important reaction as the products particularly, 2,6-di-isopropylnaphthalene can be used as a new raw material for the production of advanced aromatic polymeric materials such as naphthalene 2,6-dicarboxylate, films and thermotropic liquid crystalline polymers and speciality polyesters [27–29]. 2. Experimental 2.1. Synthesis of B-SBA-15 catalysts The B-SBA-15 was synthesised by conventional hydrothermal method using poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (P123, average molecular weight 5800, EO20PO70EO20, Aldrich), tetraethyl orthosilicate (TEOS, Aldrich) and boric acid (BDH) as template, silicon source and boron source, respectively. The composition of the gel is TEOS:xB2O3: 0.16P123: 0.46HCl. In a typical synthesis, 8 g of P123 was finely dispersed in 60 ml of water by stirring for 4 h at room temperature. Then, 240 ml of 2 N HCl solution was added with stirring, which was continued for an hour. After getting a homogeneous solution, the temperature was raised to 40 °C and the silica source TEOS (9.6 ml) was added slowly with stirring. Calculated amount of H3BO3 in order to get the gel with SiO2/B2O3 ratio of 50, 27, 20 and 10, was added as boron source. The gel mixture thus obtained was continuously stirred for another 24 h at 40 °C. Finally, the gel was transferred to Teflon cup and autoclaved at 120 °C for 72 h. After cooling to room temperature, the solid product was filtered, washed and dried at 100 °C. For comparison purposes, siliceous SBA-15 was synthesised using the same gel composition without boron at 100 °C for 48 h. The calcination of the as-synthesised materials was carried out at 500 °C with heating rate of 3 °C/min in air for 6 h to remove the template molecules. The SiO2/B2O3 ratio in gel of B-SBA-15 catalysts is given between parentheses. 2.2. Characterisation of catalysts The powder X-ray diffraction patterns of the calcined SBA-15 and B-SBA-15 were collected on a X’Pert PROXRD diffractometer using Cu Ka (k = 0.1541 nm) radiation with Ni filter. The diffractograms were recorded in the range of 0.5–10° with a step size of 0.01 and a step time of 10 s.

I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

The d-spacing (d) and unit cell parameter (a0) of the materials were calculated using the equations 2d sin h = nk and p a0 = 2d100/ 3, respectively. The elemental analysis of the as-synthesised and calcined B-SBA-15 samples was carried out on Perkin–Elmer ELAN 5000 ICP-MS instrument. The textural characteristics of the calcined SBA-15 and B-SBA-15 materials were analysed by nitrogen adsorption method. Nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics 2000 ASAP analyser. All the samples were initially degassed at 200 °C for 2–3 h under vacuum in the degas port of the analyser. The specific surface area was calculated following the BET procedure. The pore size distributions of the materials were calculated from the desorption branch of the nitrogen isotherm following the BJH method. The framework vibrations of the calcined materials were recorded on a Perkin– Elmer FT-IR spectrometer (Spectrum GX) using KBr pellet technique. The structure and morphology of the calcined SBA-15 and B-SBA-15 materials were imaged on JEOL 840A scanning electron microscope. The samples were first coated with carbon and then with gold and subjected to imaging at different magnifications. The coordination environment of boron in the hydrated and dehydrated B-SBA-15 samples was analysed by 11B MAS NMR and 11 B MQMAS NMR. The sample was evacuated to 104 Torr at 200 °C and then held at that temperature for 4 h. The sample is then cooled to room temperature and closed with cap while the evacuation was maintained. The solidstate NMR investigations were performed on a Bruker MSL500 spectrometer at resonance frequency of 500 MHz and 160.96 MHz (11.74 T) for 1H and 11B nuclei, respectively, and using a Bruker 4 mm MAS probe. The sample spinning rate was 15 kHz. The 1H spectra were measured by accumulating 64 scans with a p/2 pulse of 3.8 ls and a relaxation delay of 10 s. 11B MAS NMR spectra were acquired with a p/2 pulse duration of 1.8 ls, repetition time of 2 s and 1680 scans. A three-pulse z-filtered pulse was applied for 11 B triple quantum MAS NMR (MQMAS NMR) experiments. The optimised three pulse widths were 2.3, 0.7 and 9 ls, respectively. Two hundred and forty scans were accumulated and the recycling delay was set to 2 s in the MQMAS experiment. The 1H and 11B chemical shifts were referenced to TMS and BF3 Æ O(CH2CH3)2, respectively. The strength and type of acid sites on calcined materials were analysed by DRIFT spectroscopy of pyridine adsorbed samples using Thermo Spectra-Tech in situ IR cell. The samples (10 mg each) were placed in the in situ cell and heated up to 400 °C with heating rate of 3 °C/min in helium flow (50 ml/min) to clean the samples. The background spectra were recorded after the samples were cooled to room temperature. Then, pyridine vapours along with helium gas was introduced into the IR cell for 30 min. Then, the pyridine flow was stopped and the physically adsorbed pyridine molecules were removed by purging with helium (50 ml/min) at 100 °C for 30 min. Finally, the DRIFT spectra were recorded in Perkin–Elmer (Spectrum GX) FT-IR spectrometer in the region of 1400–1600 cm1.

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2.3. Catalytic studies The isopropylation of naphthalene (NH) with isopropyl alcohol (IPA) was carried out over calcined SBA-15 and BSBA-15 catalysts at atmospheric pressure using a fixed-bed down flow reactor in the temperature range of 250–400 °C in steps of 50 °C with cyclohexane (CH) as solvent. A mixture of NH, IPA and CH in the molar ratio of 1:2:20 was used as feed. All the catalytic runs were carried out using 1 g of catalyst. The catalyst powders were pressed without any binder, crushed and sieved to obtain particles with 16–46 mesh size. Then the catalyst was diluted with SiC (16 mesh) and packed in the reactor and activated at 500 °C for 6 h with nitrogen flow (30 ml/min). The feedstock was fed into the reactor using a syringe pump at a predetermined rate. The product mixture was collected at a time interval of 1 h at cold condition. Experiments were carried out at different temperature, feed space velocity and molar ratio of NH to IPA. The composition of the product mixture was analysed by gas chromatograph (Varian 3400) equipped with FID and a capillary column. Further, the products identification was facilitated by GC–MS (VG Analytical). 3. Results and discussion 3.1. Characterisation of catalysts 3.1.1. X-ray diffraction The low angle X-ray powder diffraction patterns of calcined SBA-15 and B-SBA-15 with different SiO2/B2O3 ratios are presented in Fig. 1. All the samples displayed a well-resolved pattern with a sharp peak at 2h value 0.8 and three small peaks at about 1.4, 1.6 and 2.1 that matched well with the reported pattern for mesoporous materials. The above peaks are indexed to the (1 0 0), (1 1 0), (2 0 0) and (2 1 0) reflections of the 2D hexagonal mesostructure with space group p6mm [5]. Presence of all the above peaks for B-SBA-15 samples confirms that the hexagonal structure is retained after the B incorporation. But, the intensity of higher order reflection peaks in B-SBA-15 samples are significantly low compared to siliceous SBA-15 indicating that the incorporation of boron in the framework slightly decreases the ordered nature of SBA-15. The important crystal parameters of the calcined materials are presented in Table 1. The d-spacing of the calcined materials are compatible with the hexagonal p6mm space group. When compared to siliceous SBA-15, the d-spacing values of B-SBA-15 materials are slightly low and further decreases with increasing boron content in the framework (decreasing SiO2/B2O3 ratio). The d-spacing values of B-SBA-15 with SiO2/B2O3 ratio 50, 27, 20 and 10 are 10.1, 9.9, 9.7 and 9.4 nm which are significantly lower than that of 10.3 nm for siliceous SBA-15. The lowering of d-spacing of B-SBA-15 samples suggests the successful isomorphous substitution of B into the siliceous framework of SBA-15 by direct synthesis method. The lowering of d-spacing is

I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

Intensity (a.u.)

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samples show a characteristic band at 960 cm1, which can be assigned to the Si–OH groups in the silanol nests [30,31]. The boron incorporation and location in the framework was revealed by a small shoulder band at 920 cm1, which is completely absent in siliceous SBA-15. This band is characteristic of tetra coordinated boron in the framework. Also, the intensity of the band at 920 cm1 is found to increase with decreasing the SiO2/B2O3 ratio. Trong On et al. [32] reported that the band at 930 cm1 is due to tetra coordinated boron in the framework of B-MCM-41. Further, a small intensity band at 1395 cm1 is observed with all the B-SBA-15 samples suggesting the presence of boron on tri coordination. Similar observation was observed in boron incorporated zeolites as well as mesoporous borosilicate materials [33,34].

(a)

(b) (c) (d) (e) 0

2

4

6

8

3.1.3. Nitrogen adsorption studies The textural characteristics of the calcined siliceous SBA-15 and various B-SBA-15 materials were analysed by nitrogen adsorption at 77 K following BET procedure. The typical adsorption isotherm of the siliceous SBA-15 and B-SBA-15 (SiO2/B2O3 = 50) materials are presented in Fig. 3a and b, respectively and the calculated important textural parameters such as BET surface area, pore volume and average pore diameter are compiled in Table 1. It is observed that all the nitrogen adsorption/desorption isotherms are of Type IV in nature as per IUPAC classification and exhibiting a H1-type broad hysteresis loop which is characteristic of large-pore mesoporous materials with narrow pore size distribution [35]. All other B-SBA-15 materials show same type of isotherm. Also, the adsorption branch of each isotherm showed a sharp inflection in the relative pressure (P/P0) range of 0.6–0.7, characteristic of capillary condensation within uniform pores. The (P/P0) position of the inflection point is correlated to the diameter of the mesopore. The sharpness and height of the capillary condensation step are the indication of pore size uniformity and the deviations from sharpness and well-defined pore filling step are the indication of increase in pore size heterogeneity. It is well known that SBA-15 materials have hexagonal arrangement of mesopores interconnected by smaller micropores. The broad hysteresis loop in the isotherms of all SBA-15 reflects the long mesopores which limit the emptying and filling of the accessible volume. Because of the incorporation of boron in the framework, the textural properties of B-SBA-15 materials are changed

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Two theta (deg.) Fig. 1. Low angle XRD patterns of SBA-15 and B-SBA-15 catalysts: (a) SBA-15; (b) B-SBA-15(50); (c) B-SBA-15(27); (d) B-SBA-15(20); (e) B-SBA-15(10).

due to the shortening of M–O bond distance by B insertion (the atomic size of boron is smaller than Si) in the framework, which brings a decrease in unit cell parameters. Similarly, the unit cell parameter a0 calculated for (1 0 0) plane is found to decrease due to B incorporation. The wall thickness of B-SBA-15 is also found to decrease significantly compared to pure SBA-15. The elemental analysis of assynthesised and calcined samples (Table 1) shows that fraction of boron was removed during the calcination. Many reports are available in the literature on the deboration of borosilicates during the calcination [24,25]. Further, in the present case, the deboration is found to be more when the SiO2/B2O3 ratio is low. 3.1.2. FT-IR spectroscopy The framework vibrations of calcined siliceous SBA-15 and B-SBA-15 materials were analysed by FT-IR spectroscopy and are presented in Fig. 2. The vibrations of SiO2 tetrahedra unit and its modification due to the incorporation of B in the framework mainly appear in the region of 500–1500 cm1. It is interesting to note that all the Table 1 Physico-chemical characteristics of calcined SBA-15 and B-SBA-15 materials Catalyst

SBA-15 B-SBA-15(50) B-SBA-15(27) B-SBA-15(20) B-SBA-15(10)

SiO2/B2O3 ratio As-syn.

Calcined

– 65 40 36 27

– 125 94 73 47.5

d100-Spacing (nm)

a0 (nm)

BET surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

Wall thickness (nm)

10.3 10.1 9.9 9.7 9.4

11.9 11.7 11.4 11.2 10.8

883 767 756 689 672

1.19 0.99 1.18 1.01 1.14

7.9 6.1 7.8 6.9 6.4

2.4 4.1 2.1 2.8 3.0

The values between parentheses are SiO2/B2O3 ratio in gel.

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Fig. 2. Framework FT-IR spectra of SBA-15 and B-SBA-15 catalysts: (a) SBA-15; (b) B-SBA-15(50); (c) B-SBA-15(27); (d) B-SBA-15(20); (e) B-SBA15(10).

Pore volume (cc/g)

Volume adsorbed (cc/g STP)

1000

800

600

6 4 2 0 0

5

10

15

Pore diameter (nm)

400

200

0

0

0.2

0.4

0.6

0.8

1

P/P0

(a)

Pore volume (cc/g)

Volume adsorbed (cc/g STP)

1000

800

600

5 4 3 2 1 0 0

5 10 Pore diameter (nm)

15

400

200

0 0

(b)

0.2

0.4

0.6

0.8

1

P/P0

Fig. 3. Nitrogen adsorption isotherm and pore size distribution of (a) SBA-15 and (b) B-SBA-15(50) catalysts.

significantly compared to siliceous SBA-15. The BET surface area of B-SBA-15 with SiO2/B2O3 ratio 50, 27, 20 and 10 is 767, 756, 689 and 672 m2/g, which are significantly lower than that of siliceous SBA-15 (883 m2/g). The fall in surface area of B-SBA-15 samples may be due to deboration during calcination, which blocks the mesopores. The structural collapse due to boron incorporation indicated by XRD analysis is also partially responsible for it. Similarly, the pore volume (1.19 cm3/g) of siliceous SBA-15 is higher than that of B-SBA-15 materials (0.99, 1.18, 1.05 and 1.14 cm3/g for B-SBA-15 with SiO2/B2O3 ratio 50, 27, 20 and 10, respectively). The average pore diameter of the B-SBA-15 samples is found to decrease with increasing B content in the framework (decreasing SiO2/B2O3 ratio). The pore size distributions of all the samples are found to be narrow and are in the range of 6–8 nm. When compared to siliceous SBA-15, the average pore diameter values of all B-SBA-15 samples are found to be significantly lower. Also, when increasing the boron content in the framework, the sharpness of condensation step is found to deviate slightly indicating the heterogeneity in pore size of B-SBA-15 at higher boron content. All the textural characteristics of B-SBA-15 samples revealed that boron incorporation slightly decreases the ordered nature of the hexagonal structure of SBA-15. Similar observation was made in XRD analysis. 3.1.4. Scanning electron microscopy The structure and morphology of the siliceous SBA-15 and B-SBA-15 samples were analysed by scanning electron microscopy. The typical SEM images of siliceous SBA-15 and B-SBA-15 with SiO2/B2O3 ratio 50, 27, 20 and 10 are presented in Fig. 4a–e, respectively. In the case of siliceous SBA-15 (Fig. 4a), it shows many rope-like domains

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I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

Fig. 4. SEM images of SBA-15 and B-SBA-15 catalysts: (a) SBA-15; (b) B-SBA-15(50); (c) B-SBA-15(27); (d) B-SBA-15(20); (e) B-SBA-15(10).

that aggregate into a microstructure with relatively uniform size, which are well agree with the previous reports [36,37]. The hexagonal structure is also noticed clearly. Each rope-like structure consists of packages of number of segmental fibers. In the case of B-SBA-15, there is no significant change in morphology when the boron content is low (Fig. 4b). But, rope-like structure of SBA-15 was changed when the SiO2/B2O3 ratio was 27, 20 and 10 (Fig. 4c–e). However, the rope-like domains with average size of 1 lm were still largely maintained. A variety of morphologies, mainly faceted rods based on elongated hexagonal prisms, are seen on samples with high boron content. 3.1.5. 1H MAS NMR and 11B MAS NMR spectroscopy Fig. 5a and b show 1H MAS NMR spectra of proton form of B-SBA-15(10) before and after dehydration, respectively. The spectrum of proton form of B-SBA-15 before dehydration consists of broad signal at 5 ppm with shoulder at 1.8 ppm. The former signal is due to water and latter is

caused by silanol group (Si–OH) [33,38,39]. Upon dehydration (spectrum b) the signal at 5 ppm disappeared and signal due to silanol group become resolved. The spectrum in Fig. 5b shows that there is a shoulder at 3.6 ppm on down field side of the silanol group signal. This signal is due to Brønsted acid sites, present in the B-SBA-15(10). The 11B MAS NMR spectra of proton form of B-SBA15(10) before and after dehydration are shown in Fig. 6. The sample before dehydration shows a broad quadrupole signal from 19 ppm to 2 ppm and a small and narrow peak around at 3.8 ppm. The broad signal at 19 ppm to 2 ppm originates from the second order quadrupole broadening of trigonally coordinated 11B nuclei present in the framework and extraframework (H3BO3) and 3.8 ppm signal originates from tetrahedral coordinated 11 B nuclei. Upon dehydration, small decrease of signal at 3.8 ppm peak and corresponding increase of signal at 19 ppm to 2 ppm indicate some conversion of tetrahedral boron to trigonal boron sites [38,39].

I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

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Fig. 5. 1H MAS NMR spectra of B-SBA-15: (a) freshly calcined sample stored in a closed vessel, no dehydration treatment; (b) after dehydration at 200 °C.

Fig. 6. 11B MAS NMR spectra of B-SBA-15: (a) freshly calcined sample stored in a closed vessel, no dehydration treatment; (b) after dehydration at 200 °C.

To better characterize the tetrahedral and trigonal boron sites, resolution enhancement was carried out by employing the 2D MQMAS NMR method. Fig. 7a and b show 11B 2D MQMAS NMR spectra of B-SBA-15(10) before and after dehydration, respectively. The shape and position of a band reveal valuable information about the quadrupole interaction of the responsible structure. For example, trigonally coordinated boron sites display bands dispersed widely along the anisotropic axis in agreement with the quadrupole parameters while the tetrahedral boron sites show rather circular bands with minor elongation along the chemical

shift axis because of nominally weak quadrupole interactions (Cqcc  0.2 MHz). The proton form of B-SBA-15 before dehydration shows two signals at 3.8 and 1.8 ppm in the chemical shift region of tetrahedral boron and a strong signal of 19–0 ppm in the chemical shift region of tri coordinated boron. According to the literature [40], signal at 3.8 ppm can be assigned to framework tetrahedral boron species having structure A as shown in Fig. 8. Signal at 1.8 ppm may probably arise from another kind of new four-coordinated boron species having structure B as shown in Fig. 8. The MQMAS NMR of dehydrated

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Fig. 7. 11B 2D MQMAS NMR spectra of B-SBA-15: (a) freshly calcined sample stored in a closed vessel, no dehydration treatment; (b) after dehydration at 200 °C.

Fig. 8. Two different tetrahedral structures of boron in B-SBA-15 framework [40].

sample shows only one signal at 3.8 ppm in the chemical shift region of tetrahedral boron and a strong signal with two shoulders in downfield side corresponding to tri coordinated boron. The tetrahedral boron site at 1.8 ppm is easily removed after dehydration at 200 °C. This site has a remarkable flexibility on the transformation between trigonal and tetrahedral coordination. Further, it was observed that there is a change in isotropic chemical shift of tri coordinated boron species from 19–0 ppm to 15–0 ppm upon dehydration. Koller et al. [38] reported that framework and extraframework trigonal boron species gave rise to isotropic chemical shifts of 9.8–10.7 ppm and 18–19 ppm, respectively. The spectrum of dehydrated sample shows that extraframework trigonal boron such as H3BO3 has been removed upon dehydration. 3.1.6. DRIFT studies of adsorbed pyridine It is well known that the incorporation of heteroatom in the neutral silica framework creates acid sites of different

type. In order to evaluate the types of acid sites in B-SBA-15 samples, DRIFT studies were carried out over pyridine adsorbed catalysts. Fig. 9 shows the DRIFT spectra of pyridine adsorbed on various B-SBA-15 catalysts. All the B-SBA-15 samples with different SiO2/B2O3 ratio give the IR bands due to pyridine adsorbed on Lewis acid sites (1455 cm1) and pyridine adsorbed on Brønsted acid sites (1541 cm1). Further, a band at 1492 cm1 is attributed to pyridine associated with both Lewis and Brønsted acid sites. The physically adsorbed pyridine is shown by a broad band at 1505 cm1. The intensity of bands for pyridine adsorbed on Brønsted and Lewis acid sites is found to increase with decreasing SiO2/B2O3 ratio, indicating that the acidity of B-SBA-15 increases with increasing boron content in the framework. Further, the 11B MAS NMR studies confirm the presence of Brønsted acid sites in B-SBA-15. 3.2. Catalytic studies Isopropylation of naphthalene with isopropyl alcohol was carried out over siliceous SBA-15 and B-SBA-15 with different SiO2/B2O3 ratio in the temperature range of 250– 400 °C in steps of 50 °C using cyclohexane as solvent. Experiments were carried out using different feed ratio (NH:IPA:CH) of 1:1:20, 1:2:20 and 1:3:20 and LHSV of 2.5, 3.5 and 5 h1. The product analysis shows that mono-isopropylated (1-isopropylnaphthalene, 1-IPN and 2-isopropylnaphthalene, 2-IPN) and di-isopropylated naphthalene (2,6-di-isopropylnaphthalene, 2,6-DIPN and 2,7-di-isopropylnaphthalene, 2,7-DIPN) are major products along with traces of cracked products. No significant activity with siliceous SBA-15 in naphthalene conversion

I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

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Fig. 9. DRIFT spectra of pyridine adsorbed on different B-SBA-15 catalysts: (a) B-SBA-15(50); (b) B-SBA-15(27); (c) B-SBA-15(20); (d) B-SBA-15(10).

was observed at all the temperatures studied due to the absence of acid sites. It is well known that this reaction is a consecutive reaction [41]. Generally, the amount of diisopropylated products is significantly lower than that of mono-isopropylated products over all the catalysts at all temperatures studied. Among the mono-isopropylated products, formation of 2-IPN was more than 1-IPN and 2,6-DIPN was more in di-isopropylated products than 2,7-DIPN indicating the isopropylation occurs preferentially at 2 and 6 position of naphthalene. The naphthalene conversion with all the catalysts at different temperatures is presented in Fig. 10. All the B-SBA-

40 B-SBA(50) B-SBA(20)

B-SBA(27) B-SBA(10)

NH convesion (wt%)

30

20

10

0 200

250

300

350

400

450

Temperature (°C) Fig. 10. Effect of reaction temperature on naphthalene conversion over different catalysts.

15 catalysts show significant activity indicating that the acid sites created by boron incorporation are responsible for the catalytic activity. It is observed that the naphthalene conversion increases with increasing reaction temperature over all the B-SBA-15 catalytic system. The increase in conversion is significantly more when the reaction temperature increased from 300 °C to 350 °C. Maximum conversion of naphthalene was observed at 400 °C over all the catalysts. Further, it is interesting to note that the naphthalene conversion increases with decreasing SiO2/B2O3 ratio (increasing the boron content in the framework). Among the B-SBA-15 catalysts, B-SBA-15(10) shows the maximum activity compared to other catalysts at all the temperatures studied. The enhanced activity of B-SBA-15(10) can be accounted in terms of the additional increase in acidity as revealed by DRIFT studies on pyridine adsorbed catalysts. The selectivity of products at different temperatures over all the boron incorporated catalysts are presented in Table 2. When considering the selectivity of the monoand di-isopropylated naphthalene products, the selectivity of monoisopropylated (1-IPN and 2-IPN) products is found to be always higher than that of di-isopropylated products for all catalysts at all the temperatures studied. It indicates that still strong acid sites are needed to produce more di-isopropylated products. Further, the selectivity of 2-IPN is found to be higher than that of 1-IPN. Only traces of 1-IPN are observed at lower temperature ranges. When increasing the temperature, the selectivity of 1-IPN and 2IPN decreases significantly over all the catalytic systems. At the same time, the selectivity of 2,6-DIPN and 2,7DIPN increases at the expense of IPNs. Also, the selectivity of DIPN is found to increase with increasing boron content in the framework. Catalyst B-SBA-15(10) shows the maximum selectivity for DIPN among the B-SBA-15 catalysts

10

I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

Table 2 Product selectivities in naphthalene isopropylation over different catalysts at different temperatures Catalyst

Temperature (°C)

Conversion (wt.%)

Product selectivity (%) 2-IPN

2,6-DIPN

2,7-DIPN

Others

B-SBA-15(50)

250 300 350 400

8.3 12.6 19.3 23.6

9.6 9.6 8.7 8.2

49.3 46.5 44.8 42.0

23.1 24.8 27.2 29.4

4.8 4.6 5.5 6.2

13.2 14.5 13.8 14.2

B-SBA-15(27)

250 300 350 400

9.5 14 19.8 28.2

10.2 9.6 8.5 7.0

47.6 44.0 40.5 40.2

23.8 26.2 27.5 29.7

4.5 5.2 6.0 6.6

13.9 15.0 17.5 16.5

B-SBA-15(20)

250 300 350 400

13 18.6 25 34.3

11.8 11.0 9.2 8.0

45.1 43.0 40.3 37.2

23.8 27.2 30.2 33.5

5.8 5.6 6.0 6.3

13.5 13.2 14.3 15

B-SBA-15(10)

250 300 350 400

14.2 18.8 27.2 35.7

8.5 7.6 6.0 6.1

45.2 41.5 38.0 36.2

25.0 29.2 34.7 34.0

6.1 6.0 6.3 6.5

15.2 15.7 15.0 17.2

1-IPN

Feed molar ratio (NH:IPA:CH): 1:2:20; space velocity—2.5 ml/h.

at all the temperature studied. Among the DIPNs, the selectivity towards 2,6-DIPN is found to be always higher than that of 2,7-DIPN indicating that isopropylation favors at 2 and 6 position in naphthalene. It is observed that 2,6-DIPN selectivity increases with increasing boron content in the framework. The catalyst with low boron content shows very low selectivity towards DIPN due to lower number of acid sites. Isopropylation of naphthalene with various NH:IPA molar ratio and feed space velocity were carried out at 400 °C over B-SBA-15(10) in order to optimize the feed molar ratio as well as feed space velocity to get maximum naphthalene conversion with high selectivity to DIPNs and the results are compiled in Table 3. The molar ratio of solvent to naphthalene is kept constant (1:20) for all the catalytic runs. It is observed that the naphthalene conversion and DIPN selectivity is affected significantly when changing the molar ratio of naphthalene to IPA and the feed space velocity. Better naphthalene conversion and higher DIPN selectivity is observed when the feed molar ratio was 1:2 with the space velocity of 2.5 h1. The naphthalene conversion increased with IPA content in the feed. This can be due to the polar nature of IPA molecule, which

Table 3 Effect of feed space velocity and molar ratio on product selectivities over B-SBA-15(10) at 400 °C LHSV (h1)

NH:IPA molar ratio

Naphthalene conversion (wt.%)

Product selectivity (%) IPNs

DIPNs

Others

2.5 2.5 2.5 3.5 5.0

1:1 1:2 1:3 1:2 1:2

29.2 35.7 37.3 28.4 23.0

47.5 42.3 30.4 42.6 39.2

27.2 40.5 38.2 28.0 22.6

25.3 17.2 31.4 29.4 38.2

competes with naphthalene for adsorption sites due to excess of alkylating agent [42]. The sustainability of the catalysts was tested by carrying out time on stream study for a period of 6 h at 400 °C under optimised experimental conditions of feed molar ratio (1:2) and space velocity (2.5 h1). All the catalysts showed a minimum fall in activity up to 3 h. After 3 h of time on stream, the activity of the catalysts falls rapidly as a result of coke deposition on the surface of catalysts, which blocks the active sites and deactivates the catalysts. 4. Conclusion Isomorphous substitution of boron in SBA-15 framework was successfully carried out for the first time by direct synthesis method. The fall in d-spacing and unit cell parameter compared to siliceous SBA-15 indicated the incorporation of boron in the framework. The SEM images of B-SBA-15 and siliceous SBA-15 showed that the morphology was changed when the boron was incorporated in the SBA-15 framework. The nitrogen adsorption studies showed that the synthesised SBA-15 materials are mesoporous in nature with narrow pore size distribution. The presence of Brønsted acid sites was confirmed by 1H MAS NMR. The 11B MAS NMR and MQ MAS NMR studies revealed that the boron in SBA-15 existed in both tri and tetra coordination even after dehydration at 200 °C. The DRIFT spectra of pyridine adsorbed catalysts claimed the presence of both Lewis and Brønsted acid sites in B-SBA-15 and the number of acid sites increased with increasing boron content in the framework. The catalytic activity of B-SBA-15 in isopropylation of naphthalene also confirmed that acid sites were created when boron was incorporated in the SBA-15 framework. Further, the increase of naphthalene conversion with decreasing SiO2/

I. Eswaramoorthi, A.K. Dalai / Microporous and Mesoporous Materials 93 (2006) 1–11

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Synthesis, characterisation and catalytic performance of ...

39.2. 22.6. 38.2. Table 2. Product selectivities in naphthalene isopropylation over different catalysts at different temperatures. Catalyst. Temperature. (°C).

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