Triflamide Anchored SBA-15 Catalyst for Nitration of Alkyl Aromatics in Microwave

Indian Journal of Chemistry Vol. 53A, April-May 2014, pp. 545-549

Triflamide anchored SBA-15 catalyst for nitration of alkyl aromatics in microwave

Venkata Siva Prasad Ganjalaa, b, Suresh Mutyalaa, Chinna Krishna Prasad Neelia, Mukkanti Khaggab, *, Kamaraju Seetha Rama Raoa & David Raju Burria, * aCatalysis Laboratory, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 607, India Email: [email protected] bCentre for Chemical Sciences & Technology, Institute of Science and Technology, Jawaharlal Nehru Technological University, Kukatpally, Hyderabad, 500 085, India Email: [email protected]

Received 20 January 2014; revised and accepted 29 January 2014

A series of triflamide anchored SBA-15 (SBA-NH-TA) catalysts with 5-20 wt% triflic acid (TA) loadings have been synthesized through the functionalization of propylamine on the surface of SBA-15 (SBA-NH2), followed by covalent attachment of triflic acid with –NH2 group of SBA-NH2. With SBA-NH-TA as catalyst and 69% HNO3 as nitrating agent, highly accelerated and safe nitration of aromatic compounds in microwave under solvent-free conditions has been achieved. The structural and textural characteristics of SBA-NH-TA catalysts have been determined from N2 sorption and low-angle XRD techniques. As the loading of TA increases, the conversion of alkylaromatics to their nitrated products increases significantly. Reaction parameters like amount of catalyst, amount of triflic acid loading, substrate to nitric acid ratio, reaction temperature and reaction time have been investigated.

Keywords: Catalysts, Nitration, Metal-free nitration, Triflamide anchored SBA-15, Alkyl aromatics, Xylene

Nitration of aromatic substrates is one of the most been studied for nitration of aromatic compounds. important and widely studied chemical reactions1-3. However, most of these catalysts have disadvantages Even today, the nitration industry relies largely on like high cost, harsh reaction conditions, low early technology which involves a mixture of nitric conversion and poor selectivity. acid and sulfuric acid. This conventional method has Mesoporous SBA-15 silica has been used as a many disadvantages like low selectivity, over potential catalyst support since its first synthesis23, 24 nitration, oxidized product formation and generation because of its interesting textural properties, such as 4 of environmentally hazardous waste . Therefore, it is large specific surface area, uniform pore size, essential to have an alternative nitration method to framework of thick walls, small crystallite size of circumvent the problems involved in the conventional primary particles and complementary textural porosity 5-7 nitration process . In this regard, an alternative including its high surface-to-volume ratio, variable method could be the replacement of sulphuric acid framework composition and high thermal stability25-27. with solid acid catalysts. Triflic acid has been found to be a very effective Nitration of the aromatic compounds using Lewis acid catalyst in a series of organic reactions different catalysts has been studied substantially in the like Diels-Alder28, 29, Friedel-Crafts30, Michael past decade. Alternative systems have been developed reaction31 and other organic synthesis32. Although the in the past years for nitration over zeolites8, 9. triflic acid is a highly efficient catalyst for various In addition to zeolites, a variety of other solid organic transformations, it has not yet been studied acid catalysts such as silica gel10, modified silica11, for the nitration of alkyl aromatics either in the bulk sulfuric acid on silica12, clay supported or anchored form. Herein, we report the preparation metal nitrates13-16, metal exchanged clays17, sulfonated of triflamide anchored SBA-15 metal-free solid acid polystyrene resin and sulfonated zirconia18, catalysts. Its characterization and catalytic application Nafion-H19, metal triflates20, metal sulfonates21, and in nitration of alkylaromatics under microwave rare earth metal cations exchanged Zeolite beta22 have conditions has also been described. 546 INDIAN J CHEM, SEC A, APRIL-MAY 2014

Materials and Methods measurement. FT-IR spectra were obtained over the -1 Preparation of triflic acid anchored SBA-15 wave number range of 4000 to 400 cm with a SBA-15 was synthesized in accordance with spectral resolution of 2 cm-1 on a Perkin Elmer FT-IR the literature procedures23, 33. A solution of spectrometer (Spectrum GX).

EO20PO70EO20:2 M HCl:TEOS:H2O = 2:60:4.25:15 Catalytic activity (mass ratio) was prepared, stirred for 24 h at 40 °C All the reactants were used without further and then hydrothermally treated at 100 °C under static purification. p-xylene (purity >99%) and nitric condition for 24 h. Subsequently it was filtered, dried acid (purity 69-72%), were obtained from Ranchem, at 100 °C and calcined at 550 °C for 8 h, to yield India. In order to scrutinize the catalytic activity of mesoporous silica SBA-15 as a white powder. the triflamide SBA-15 catalyst for nitration of Prior to the functionalization procedure, the alkylaromatics by microwave under solvent-free parent SBA-15 was dried in an oven at 120 °C conditions, p-xylene was taken as a model overnight under vacuum, to remove any physisorbed substrate. In a typical procedure, 1 mmol of p-xylene, water. In a post synthetic approach, 1 g of degassed 1.5 mmol of nitric acid (69%) and 20 mg of SBA-15 was dispersed in 50 mL of dry toluene under catalyst were taken in a 10 mL reaction vessel. N2 atmosphere, then 3 mL of 3-(aminopropyl)- The product analysis was carried out using a gas triethoxysilane (APTES) was added and the chromatograph (GC-17A, Shimadzu Instruments, mixture was refluxed for 24 h under anhydrous Japan) with an Equity-5 capillary column conditions. The solid product was recovered by (0.53 mm × 30 m) with FID detector. The products filtration under vacuum, washed with 50 mL of were confirmed using a GC-MS (QP-5050A model, toluene and dried at 120 °C in an oven for 12 h, Shimadzu Instruments, Japan) equipped with a designated as SBA-NH2. DB-5 capillary column (0.32 mm dia. and 25 m Triflamide SBA-15 (SBA-NH-TA) was length, J&W Scientific, USA). prepared through anchoring of triflic acid on the surface of amine functionalised mesoporous Results and Discussion silica (SBA-NH2) as shown in Scheme 1. Typically, to Low-angle X-ray diffraction patterns of the 1 g of SBA-NH2, requisite amount of triflic acid functionalised SBA-15 catalysts along with the parent loading was added in dry toluene solution under SBA-15 are shown in Fig. 1, which exhibited three N2 atmosphere and refluxed overnight. It was typical diffraction lines at 0.91°, 1.57° and 1.82° then filtered, washed with ethanol to remove respectively on the 2θ scale that are indexable as unanchored triflic acid and dried at 100 °C to yield (100), (110) and (200) reflections associated with the triflamide SBA-15. p6mm hexagonal symmetry.

Characterization of catalysts X-ray diffraction (XRD) patterns of the catalyst samples were recorded at room temperature on an X-ray diffractometer (Multiflex, Rigaku, Japan) with a nickel filtered Cu Kα radiation. N2 adsorption- desorption isotherms were recorded on a N2 adsorption unit at -196 °C (Quadrasorb-SI V 5.06, Quantachrome Instruments Corporation, USA). The samples were out-gassed at 150 °C for 4 h before the

Fig. 1—Low-angle XRD patterns of SBA-15 and its functionalized samples. GANJALA et al.: TRIFLAMIDE ANCHORED SBA-15 CATALYST FOR NITRATION OF ALKYL AROMATICS 547

–1 Figure 2 shows the N2 adsorption-desorption intensity of the IR bands at 1085, 805, and 462 cm isotherms of triflamide catalysts together with that compared with SBA-NH2 may be considered as proof of pure SBA-15 . The isotherms obtained for pure for the presence of TA in the channels of SBA-NH-TA. SBA-15 and functionalized catalysts are of type The increase in triflic acid loading was confirmed IV and exhibited a hysteresis loop of H1 type and a by CHNS elemental analysis through the estimation sharp capillary condensation step in the P/P0 range of of sulphur content (Supplementary Data, Table S1). 0.6−0.8, which is a characteristic of large channel-like Figure 4 shows the thermogravimetric analysis pores with a narrow pore size distribution (PSD). (TGA) curves of functionalized SBA-15, SBA-NH2 The surface areas were calculated by using the BET and SBA-NH-TA samples. For all the three samples, model and are shown in Table 1. The total volumes of a marginal weight loss is observed between 90 and mesopores were calculated from the amounts of 120 oC. This is mainly due to the loss of physically nitrogen adsorbed at P/P0 of 0.98, assuming that adsorbed water molecules. In the case of SBA-NH2, a adsorptions on the external surface were negligible major weight loss is observed from 240−450 oC, compared with the adsorption in pores. The PSD of which is due to thermal decomposition of the grafted all the calcined catalysts are shown in Fig. S1 aminopropyl triethoxysilane moiety. The major (Supplementary data). There is a decrease in the total pore weight loss is observed from 280–500 oC in the case volume of the triflamide SBA-15 sample as compared of triflamide SBA-15. From the above results, it is with that of pure SBA-15 due to functionalization of concluded that the triflic acid moiety was covalently triflic acid on the framework of SBA-15. anchored on the surface of amine functionalized SBA-15. The FT-IR spectra of SBA-15, amine SBA-15 Table 1—Textural and structural parameters of functionalized (SBA-NH2), triflamide SBA-15 (SBA-NH-TA) and Triflic acid (TA) are depicted in Fig. 3. The bands at SBA-15 samples obtained from N2 sorption data –1 a b c 1630 cm may be assigned to the OH vibrations of No. Catalyst SBET Vt d (m2/g) (cc/g) (nm) physisorbed H2O. The Si–O–Si bands originate from SBA-15 and are observed around 1085, 805, and 1 SBA-15 697 1.043 13.20 –1 –1 462 cm . The band at 972 cm is assigned to the 2 SBA-NH2 359 0.637 9.13 vibration of the silanol groups (Si–OH), while the 3 5SBA-NH-TA 305 0.599 9.10 bands at 1551, 3270 cm-1 correspond to bending and 4 10SBA-NH-TA 276 0.464 7.98 5 15SBA-NH-TA 217 0.406 7.90 stretching vibrations of the N–H bond. These results 6 20SBA-NH-TA 199 0.343 7.10 confirm the successful functionalization of SBA-15 7 SBA-TA 556 0.898 10.78 with aminopropyl groups34. The SBA-NH-TA sample a BET surface area; b Total pore volume; c BJH pore diameter. showed four characteristic bands at 1470 cm–1 (C-N), –1 –1 769 cm (C-S), 645cm (>SO2). The appearance of new band at 643 and 769 cm–1 and an increase in the

Fig. 2—N2 adsorption-desorption isotherms of SBA-15 and its functionalized samples. Fig. 3—FTIR spectra of SBA-15 and its functionalized samples. 548 INDIAN J CHEM, SEC A, APRIL-MAY 2014

Table 2—Nitration of p-xylene over different catalysts. [(HNO3)/(p-xylene) mole ratio = 1.5; amt of catalyst = 20 mg; reaction time = 5 min] No. Catalyst Conv. Selectivity (%) (%) (2-NO2) Oxidised product product 1 SBA-15 68 90 10 2 SBA-NH 47 87 13 2 Fig. 4—TGA curves of SBA-15 and its functionalized samples. 3 SBA-TA 75 85 15 Catalytic activity 4 5SBA-NH-TA 87 93 07 The nitration of alkyl aromatics over triflamide 5 10SBA-NH-TA 95 99 01 SBA-15 catalyst has been schematically presented in 6 15SBA-NH-TA 97 94 06 Scheme 2. To understand the nature of triflamide 7 20SBA-NH-TA 100 92 08 SBA-15 catalyst, initial experiments were conducted using p-xylene as a substrate and 69% HNO3 as a nitrating agent under solvent-free conditions with microwave irradiation. Subsequently, the reaction parameters like p-xylene to HNO3 ratio, amount of catalyst and reaction time were optimized (Supplementary Data, Tables S2-S4). As per the results obtained, it is found that the optimum reaction time is 5 min, amount of catalyst is 20 mg and p-xylene:HNO3=1:1.5. To authenticate the superior activity of SBA-NH-TA catalyst of various loadings, the amine SBA-NH2 and SBA-15 materials were also studied and their activity details are presented in Table 2. In order to check the recylability of 10SBA-NH-TA catalyst, the catalyst was isolated from the product mixture by centrifugation, washed with acetone, dried at 120 °C for 6 h and re-used under the above mentioned optimized reaction conditions. It is found that there is no considerable efficiency loss in activity for at least 3 repeated cycles. The scope of SBA-NH-TA catalyst for the nitration of aromatic compounds other than p-xylene has also been verified taking different aromatic compounds such as benzene, ethylbenzene, ortho, meta xylenes and chlorobenzene. It is found that all the substrates were smoothly converted into their respective mononitrated products with traces of oxidized products.The details of the different substrate conversions under optimized conditions are displayed in Table 3. aMononitro product. GANJALA et al.: TRIFLAMIDE ANCHORED SBA-15 CATALYST FOR NITRATION OF ALKYL AROMATICS 549

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