Highly selective synthesis of cyclododecanone over mesostructured VSBA-15 catalysts

Abstract

Highly ordered VSBA-15 mesoporous catalysts have been synthesized under pH-adjusting direct hydrothermal (pH-aDH) method using vanadyl sulphate hydrate and Pluronic P123 as the sources of vanadium and template, respectively. The mesoporous catalysts characterized by sophisticated instrumental techniques, viz. ICP-AES, XRD, N₂ adsorption, ESR, UV–vis DRS, ⁵¹V MAS NMR, ²⁹Si MAS NMR and TEM show their two-dimensional mesostructures with tetrahedral vanadium species on the silica surface. The well ordered VSBA-15 catalysts have been used in the oxidation of cyclododecane (CDD) with hydrogen peroxide (H₂O₂, 30%) to find their catalytic activities. The regenerated VSBA-15 catalysts have been examined to find their catalytic stabilities. VSBA-15(5) catalyst has been washed with ammonium acetate solution to investigate the leaching of vanadium species in the framework of silica network of SBA-15, and the catalytic activity of washed VSBA-15(5) has also been examined in the catalytic reactions. Moreover, the hydrothermally stable VSBA-15(5) catalyst has also been examined in the catalytic reaction to find the effect of its catalytic activity. Additionally, the influences of various reaction parameters such as temperature, time, ratios of reactant and solvents on the oxidation of CDD have been investigated. Based on the catalytic results, VSBA-15(5) catalyst is found to be a highly active, recyclable and promising heterogeneous catalyst for selective synthesis of cyclododecanone (CDDO).

Introduction

Vanadium-containing microporous catalysts synthesized using different hydrothermal conditions were used in several oxidation reactions to produce a variety of selective products [1], however, their catalytic activities strongly depend on the syntheses methods [2], [3]. The microporous catalysts restrict the accessibility of large substrate molecules due to their smaller pore sizes. The described limitations have been overcome by the discovery of mesoporous M41s materials in 1992 [4]. Since Beck and co-workers reported the mesoporous MCM-41 synthesized using direct hydrothermal method [4], extensive research efforts have focused in the catalytic applications of heteroatoms containing mesoporous silicas [5], [6], [7], [8]. Highly ordered mesoporous VMCM-41 materials were successfully synthesized and used in the oxidation of aromatics [6], [7], [8], [9], [10], [11], [12], [13], [14], and they gave good catalytic activities with good selective products. In 1998, Zhao et al. reported a new family of mesoporous SBA-15 synthesized under acidic hydrothermal condition [15]. Particularly, all the mesoporous materials (MCM-41 and SBA-15) have received much attention from researchers in the past decade because of their potential applications such as catalysts, adsorbents and guest–host chemical supports for large organic molecules [5], [6], [7], [8], [16], [17], [18]. However, SBA-15 has better hydrothermal stability than MCM-41 due to thicker pore walls [4], [15], and it has also larger pore size than MCM-41. But, the pure SBA-15 material is unable to be used directly as a catalyst due to the lack of acidities. Hence, it has reported that the guest species like heteroatoms or organic moieties can be introduced into the framework of SBA-15 to increase the active sites and thus improve the catalytic activity [16], [17]. Therefore, it is of great importance to introduce heteroatoms into mesoporous silica materials prepared under strongly acidic conditions. Several research reports are available, as the heteroatoms (M = Al, Fe, V, Ti and Ga) were incorporated in the framework of SBA-15 by direct synthesis or post-synthetic grafting method, for creating the active sites on the silica surface, and the synthesized materials were tested for their catalytic activities for a certain reaction [19], [20], [21], [22], [23], [24]. As mentioned in a previous report [25], it is difficult to incorporate the high amounts of heteroatoms into the mesoporous silica walls under the highly acidic conditions, and the introduced metals exist in the cationic form rather than as their corresponding oxo species. Most heteroatoms introduced by post-synthesis procedure may be located with octahedral coordination in the resultant materials [21], [24], [26]. On the basis of such a serious limitation, the mesoporous MSBA-15 (M = metal species) materials synthesized by different hydrothermal conditions were reported [19], [22], [23], [25]. It was also found that the MSBA-15 mesoporous materials have highly active sites on the silica pore walls from the observation of their characterization results. The MSBA-15 materials were widely used as the catalysts in the several catalytic applications [19], [20], [21], [22], [23], [24]. Particularly, Selvaraj and his research group successfully reported the mesoporous MSBA-15 materials (M = Cr, Mn, Sn, Ti, Ga, Al and Nb) synthesized using pH-adjusting direct hydrothermal (pH-aDH) method [27], [28], [29], [30], [31], [32], [33], [34], [35]. From these characteristic results, one can conclude that MSBA-15 catalysts have higher hydrothermal stabilities than MMCM-41 catalysts. The MSBA-15 catalysts were used in different catalytic reactions to produce the selective fine chemicals [28], [30], [31], [32], [34], [35].

In 2004, Hess et al. reported the highly dispersed vanadia on SBA-15 synthesized using organic moiety functionalized vanadium precursor [36]. Recently, Gao et al. reported the VSBA-15 synthesized using ammonium metavanadate as a vanadium source under different pH values [37]. Du et al. reported the VSBA-15 prepared using oxovanadium (V) chloride as a vanadium precursor using atomic layer deposition method [38], and the synthesized VSBA-15 were used in the oxidation of methanol to produce formaldehyde with a good selectivity. Jurado et al. reported the selective epoxidation of alkenes using highly active VSBA-15 catalysts, which were synthesized using vanadium oxytriisopropoxide and ammonium metavanadate as the sources of vanadium [39]. However, to the best of our knowledge, the highly ordered two-dimensional VSBA-15 catalysts synthesized by pH-adjusting direct hydrothermal (pH-aDH) method using vanadyl sulphate hydrate as a vanadium precursor have not been clearly reported with enhanced hydrothermal stabilities and catalytic activities, in the open literature, and the removal of V₂O₅ crystallites on the surface of catalysts is not openly reported.

Herein we report the two-dimensional mesoporous VSBA-15 catalysts synthesized by pH-aDH method. To investigate the mesoporous nature with the environments of vanadium species on the silica network of pore walls, the synthesized VSBA-15 catalysts have been characterized by ICP-AES, XRD, N₂ adsorption, ESR, UV–vis DRS, ⁵¹V MAS NMR, ²⁹Si MAS NMR and TEM. Moreover, the catalytic activities of VSBA-15 catalysts have been also investigated after chemical and hydrothermal treatments. The calcined VSBA-15 catalysts have been used for the synthesis of cyclododecanone (CDDO) by the liquid-phase oxidation of cyclododecane (CDD) with H₂O₂ under different optimal conditions.