Ultrasound assisted improved synthesis of TiO₂ catalyst and subsequent evaluation for isomerization of alpha pinene

Highlights

• Improved synthesis of catalyst based on the use of ultrasound.

• Detailed characterization of the catalyst to highlight improvements.

• Demonstration of intensified activity in subsequent isomerization reaction.

• Ultrasound assisted synthesis yields stable catalyst with lower size and higher acidity.

• Desirable product distribution obtained based on the use of catalyst.

Abstract

Titanium dioxide was synthesized using improved method involving sonication and acid activation with comparison of activity with the catalyst synthesized using conventional approach. Studies related to effect of sonication time (5 to 25 min) on the catalyst particle size revealed that sonication drastically reduced the particle size. The catalysts were also characterized by SEM, XRD, FTIR, TGA and TPD methods. The catalyst efficacy was evaluated for isomerization of alpha pinene and the results revealed that 586 nm as the particle size of catalyst obtained using sonochemical synthesis was optimum to achieve maximum conversion of alpha pinene with good selectivity towards bicyclic monoterpenes. At optimized conditions of 0.6% catalyst loading, 586 nm as particle size of catalyst and 40 min of reaction time, alpha pinene was completely isomerized with high selectivity towards bicyclic monoterpenes (camphene, tricyclene and carene). Catalyst reusability study established that the reactivated catalyst showed excellent stability and reusability up to 8 cycles. Overall an improved process for catalyst synthesis based on ultrasound with successful subsequent use in the isomerization reaction has been demonstrated in the current work. The role of ultrasound was clearly established in terms of reduction in particle size and increased number of acidic sites that drive excellent catalytic performance.

Introduction

In the recent years, there has been a huge shift towards manufacturing of high value added chemicals from the bio-feedstock in a sustainable manner. One of the important inexpensive feedstock is turpentine derived from plant materials such as pine trees which can be converted to alpha pinene that forms an important feedstock for the synthesis of various fine chemicals. Alpha pinene is very reactive chemical and can be easily isomerized to different bicyclic and monocyclic monoterpenes in the presence of acid catalyst. The various products formed during the isomerization reaction of alpha pinene are camphene, tricyclene, dipentene, carene, terpinene, terpinolene and p-cymene [1]. All these compounds can be consumed directly or used as feedstock in different industries such as food, perfumery, pharmaceuticals, fine chemicals and paint [2]. Isomerization of alpha pinene is a very important process and generally carried out in the presence of titanium dioxide catalyst [3,4]. But this process suffers major limitations of slower reaction rate, longer reaction time (greater than 24 h), deactivation of the catalyst and low selectivity towards bicyclic monoterpenes. Considering this limitation, lot of interest has been on the development of new catalysts which can result in high catalytic performance and better selectivity towards bicyclic monoterpenes, specially tricyclene and camphene, within a short time of reaction. Tricyclene and camphene are vital intermediates for the production of many valuable chemicals including camphor and have high market value. Camphene is also used for the synthesis of toxaphene insecticide [5], terpenecyclohexanols (used as an alternative to sandalwood oil) and toilet fragrances [6].

Finding a catalyst having excellent catalytic activity and selectivity towards high value products during the alpha pinene isomerization is extremely difficult. Various heterogeneous catalyst have been tested for the isomerization of alpha pinene namely, zeolites [5,[7], [8], [9], [10], [11]], sulphated zirconia [12], [13], [14], clays [15], [16], [17], [18], [19], W₂O₃-Al₂O₃ [20], resins [2,21], HPW-SBA-15 [3], Al- MCM 41 [22,23], solid super acids [24], Ti-M [25,26], rare earth oxides [27], heteropoly acids [28], H-mordenite molecular sieves [4], GA-SBA 15 [29], acid activated natural aluminosilicates [30,31] and Ti-SBA-15 [32], titanium nanotubes[33], dealuminated ferrite-type zeolite [34] and 2D Ti3C2Tx MXene [6]. During the application of catalysts in isomerization reaction to check the efficacy of catalyst, the effect of reaction conditions has also been studied. For example, the isomerization reaction over acid treated montmorillonite clays was studied by Yadav and co-workers [17] and greater than 96% alpha pinene conversion was reported with 39-49% camphene selectivity under best reaction conditions. Rachwalik et al. [35] studied the transformation of monoterpenes using ferrierite type zeolite and reported that camphene and limonene were the main products formed during the isomerization reaction with selectivity of 90% within 180 min of reaction. Dziedzicka et al. [36] studied the liquid phase alpha pinene isomerization using modified natural zeolite catalyst and reported 95% conversion of alpha pinene in 120 min of reaction with the selectivity of camphene and limonene as 40% and about 32%, respectively. Gackowski and co-workers [37] investigated the catalytic performance of Hierarchical zeolite mazzite catalyst for the isomerization of alpha pinene and it was reported that 40% and 37.1% selectivity of camphene and limonene was achieved at 44% conversion of alpha pinene in 120 min of reaction. Huang et al. [33] investigated the isomerization of alpha pinene using titanium nanotubes and reported 100% conversion of alpha pinene and 78.5% camphene selectivity at reaction conditions of 120°C, 120 min of reaction time and using 1 ml of alpha pinene. Liu et al. [4] studied the isomerization reaction over hierarchical mordenite molecular sieve synthesis using alkaline treatment assisted by microwave resulting in 93.4% alpha pinene conversion with 34.7% camphene and 11.9% limonene selectivity. Zielińska et al.[6] and Wróblewska et al. [32] demonstrated the isomerization of alpha pinene over exfoliated Ti₃C₂T_x and Ti- SBA-15 respectively and reported that at 100% conversion of alpha pinene, the selectivity of camphene was 60 mol% and 26 mol% respectively.

In spite of many studies reporting different catalysts for isomerization, there is still continued interest in developing new catalysts which can exhibit high catalytic performance and selectivity towards camphene during the isomerization of alpha pinene. It is worth noting that, the time required for complete conversion of alpha pinene resulting in desired product selectivity depends on the amount and nature of the catalyst [5]. Typically the use of titanium dioxide for the isomerization reaction of alpha pinene, though simple, offers drawbacks as lower rate of reaction and low selectivity of camphene and transformation towards other bicyclic monoterpenes as compared to dipentene formation which is the major limitation as dipentene has comparatively low market value [33]. Surface modification of titanium dioxide catalyst with acid treatment have been reported as an efficient method to increase the catalytic performance. Sonication is also applied widely during the synthesis process to improve the properties of the catalyst [38], [39], [40] though has not been reported for the catalyst used in isomerization reaction. The cavitational effect of sonication is well known to control the nuclei formation which enables the formation of smaller particles and it also favours the deagglomeration of particles thus reducing the particle size [41], [42], [43]. For example, Taurozzi and co-workers [43] studied the ultrasound assisted dispersion of nanoparticles and reported that when ultrasonication is provided to the catalyst cluster in the liquid suspension, the fragmentation can easily occur either by the fracture or erosion of the surface of the catalyst. The fragmentation of particle can occur in isolation or simultaneously depending on the characteristics of the catalyst powder and the energy provided. Shi et al. [44] investigated the ultrasonic precipitation for synthesis of Ni/Al₂O₃ catalyst and reported that the application of sonication resulted in the reduced particle size of 6.79 $\mu$m as compared to 13.91$\mu$m as the particle size obtained in the conventional method. Bathula et al. [42] studied the green synthesis of palladium nanoparticles driven by ultrasonication to be applied for the Suzuki-Miyaura reaction and catalytic reduction. It was reported that the catalyst particles synthesized were spherical shaped and less aggregated.

In the present study, we aim to synthesize the titanium dioxide catalyst by the application of ultrasound coupled with activation using base and acid treatment so as to enhance the catalytic performance. To the best of our knowledge, this is first study where catalyst was synthesized using sonication, further activated with base and acid treatment and employed in isomerization of alpha pinene. The catalyst was also synthesized using the conventional approach without the use of ultrasound to establish the beneficial effect of the sonication. The catalysts were characterized using different techniques like SEM, XRD, FTIR, TGA and TPD so as to get the fundamental information for the catalyst and establish the possible role of ultrasound in improving the catalyst characteristics. The work also attempts to establish conditions to achieve higher reaction rate with improved bicyclic monoterpene (carene, tricyclene and camphene) selectivity. The influence of different parameters such as particle size, time and catalyst dosage on the alpha pinene isomerization and selectivity were hence evaluated. Additionally, the optimized catalyst was tested for reuse in multiple cycles in both approaches as with and without reactivation.