Synthesis of Hierarchical Micro-mesoporous ZSM-5 Zeolite and its Catalytic Activity in Benzylation of Mesitylene

Hierarchical micro-mesoporous ZSM-5 (MFI type) zeolite was synthesized by using corn plant stem pith powder from agricultural waste as hard template under simple hydrothermal method. The additional porosity generated using this pith powder into the zeolites precursor gel yielded hierarchical micro-mesoporous ZSM-5 zeolite (C-ZSM-5). Conventional micro porous ZSM-5 (ConvZSM-5) was also prepared by same procedure without adding the corn stem pith powder to compare the catalytic performances. The prepared C-ZSM-5 zeolite exhibited 89% conversion, much greater than ConvZSM-5 (33%). Selectivity in the formation of 2-Benzyl-1, 3, 5-trimethylbenzene, by benzylation with C-ZSM-5 (70%) is more than two times than ConvZSM-5 (32%). This benzylation can be used as a test reaction to evidence the formation of hierarchical micro-mesoporous nature of the material. C-ZSM-5 exhibited better selectivity and benzyl alcohol conversion due to the presence of hierarchical pores and strong acidy in C-ZSM-5.The product formed in the above reaction was separated and further confirmed by 1HNMR and 13CNMR. Even after consecutive recycles every month for three months, the conversion and desired products selectivity had not been affected. The hierarchical pores present in the catalyst further improved the stability of the zeolites. Hence this catalyst could be very useful for industrially important reactions.


Introduction
Zeolites are crystalline aluminosilicate materials having pore diameters less than 2 nm, mainly comprising of (AlO 4 ) 5− and (SiO4) 4− tetrahedra (known as TO4) units bonded together which help them to make 1D,2D or 3D pore networks containing pores or voids or channel formations [1,2]. It was used in different fields like adsorption, separation, catalysis and ion-exchange etc. [3][4][5][6][7]. It was important to note that acid, base and oxidation reactions, catalyzed by zeolites, are also known. So far, more than 248 framework zeolites were reported but only few types of zeolites like FAU, ZSM-5 and BEA are commercially synthesized by various methods and used in different catalytic applications [8][9][10][11]. The zeolite ZSM-5 (Zeolite Socony Mobil-5) is a MFI type, medium pore zeolite and it belongs to the pentasil family [12]. One of the important drawback or limitations of zeolite was the presence of micropores which hinders the accessibility of bulky or large sized molecules to the acidic sites present in the pores or voids. Thus, there is a necessity to overcome this drawback by creating the meso and /or macropores. The zeolites with additional porosity are termed as hierarchically porous zeolites [13][14][15].  have used the seeded growth procedure to synthesise hierarchically structured silicalite-1 zeolite using bamboo or cedar woods as sacrificial template [16]. In the search for a suitable hard template for the preparation of hierarchical zeolites several materials have been tried. Carbon particles are often used as hard templates to prepare zeolites with additional porosity, because they can be easily burned out after preparation to generate additional pores in the zeolites [17]. Biological materials such as plant, eggshell membrane, wood, and bamboo are some of the templates which have been used for the preparation of hierarchical zeolites [18][19][20][21]. Li et al. 1 3 (2005) reported the preparation of hierarchically structured silicalite-1 monoliths using resorcinol formaldehyde based aerogel as a template. This hierarchical silicalite-1 showed improved catalytic activity in Beckmann rearrangement of cyclohexanone oxime [22]. During the crystallization process of zeolites, the carbon materials are incorporated into the zeolites and produce larger pores upon their removal during calcination. Sucrose can be used as a template for preparation of mesoporous zeolites. Carbonization of sucrose with zeolite precursors followed by crystallization and removal of the carbon can create meso porosity [23]. In our prior studies, we have successfully prepared the heteropoly acid (12-phosphotungstic acid) supported silicate-1 with intracrystalline nano voids using biomass (corn stem pith powder) as templates and showed better catalytic performance in esterification of levulinic acid with various alcohols. Silicate-1 is pure form of silica which was used as a support for hetero poly acid [24].
In continuation of above research work, we tried to make other framework type of zeolites (ZSM-5) using corn stem power as cheap, low cost hard template and to check its suitability for other reactions.In the present study, we have prepared hierarchical micro-mesoporous ZSM-5 zeolites using corn stem pith power as hard template (C-ZSM-5) and probed its catalytic performances in mesitylene benzylation using benzyl alcohol to prove the existence of mesopores in zeolites.

Synthesis of ZSM-5 Zeolites with Hierarchical Pores
The starting materials, used to make ZSM-5 zeolite with mesopores and characterization techniques, are given in supporting information (Section 1, 2).The synthesis of ZSM-5 with hierarchical micro-mesopores is as follows. Initially, Corn plants obtained from agricultural waste were dried under sunlight and made to into powder form. Poly (diallyldimethylammonium chloride) (PDDA, 1%) was used to change the surface charge of the hard templates [24]. 1 g of corn plant stem pith power was taken in the 100 ml beaker and followed by addition of PDDA solution and stirred for 2 h after prefixed time. Filtered PDDA modified pith powder from the solution was added into the zeolite precursor gel containing 7 g of Tetraethylorthosilicate (TEOS), 8 ml Tetrapropylammoniumhydroxide(TPAOH), 0.115 g sodium aluminate, 2 g ethanol, and 2 g double distilled water and the reaction mixture was continuously monitored and stirred for 2 h. Once the above process completed, the reaction mixture, placed in100 ml Teflon lined autoclave was kept in the oven for 36 h at 170℃. After prefixed crystallization time, the autoclave was brought to room temperature. The zeolite precursor gel molar ratio of was 30 SiO 2 : 7.3 TPAOH: 400 H 2 O: 5 EtOH: 0.02Al 2 O 3. The silica/alumina ratio was 25.The solid material, obtained, was washed and dried at 110 °C overnight and templates were removed by calcination process at 550 °C under air. Finally, the Na form of ZSM-5 zeolite powder obtained was treated three times with 1 M ammonium nitrate (NH 4 NO 3 ) solution at 70 °C and stirred for 5 h. The TPAOH/Si molar ratio zeolite gel was maintained at 0.25. The ion-exchange process was repeated three times for complete ion exchange and then the material was thoroughly washed with distilled water. The ammonium form zeolite powder thus obtained was then dried at 80 °C for 6 h and then calcined at 550 °C for 5 h in air to get H + form zeolites. The zeolites prepared using corn plant named as C-ZSM-5. H-ZSM-5 zeolite without adding hard template was named as ConvZSM-5.

Results and Discussions
The powder XRD patterns of prepared ZSM-5 are depicted in supporting information (Fig. S1).The prepared materials exhibited the characteristic Bragg reflections corresponding to MFI framework types, which confirmed the formation of zeolites. The diffraction peaks (at 2 theta) 8, 8.5, 22, 23, 24 were well matched with standard ZSM-5 zeolites [25]. In addition, the framework characteristics of prepared materials were analysed by FT-IR spectra (Fig. S2). The bands at 550 cm −1 is mainly related to the double five-membered ring vibration in MFI zeolite and 457 cm −1 is due to the T-O bending vibration, which confirms the MFI topology [26]. Nitrogen adsorption and desorption isotherms of the prepared ZSM-5 zeolites are illustrated in Fig. 1a. Non-local density functional theory (NLDFT) pore size distribution curves of ZSM zeolites and Calculation Model: N 2 at 77 K on silica (cylinder. pore, NLDFT adsorption branch model) are shown in Fig. 1B. In the ConvZSM-5 zeolites, hysteresis loop seen at p/p 0 ≈ 0.2 may be due to the transition of fluid to crystalline phase of nitrogen molecules, which occurs in zeolites of MFI type. The results clearly show that the prepared zeolites are well-crystallized under these reactions conditions and it coincides with XRD data. In addition to the hysteresis at p/p 0 ≈ 0.2, a second hysteresis, observed above p/p 0 ≈ 0.4, in C-ZSM-5indicates the formation of additional porosity with hierarchical micro-mesopores. The broadness and shape of this isotherm reveal the presence of cage like type of pores in addition to intrinsic micropores of zeolites. However it is noteworthy to see that this additional porosity is not observed in ConvZSM-5 zeolites.
The Non-local density functional theory (NLDFT) pore size distribution curves of ZSM-5 zeolites pore size distribution data, shown in Fig. 1B, confirm the presence of these additional pores in C-ZSM-5. Further, it also established that the additional pores are absent in ConvZSM-5 and the external surface area of zeolites increases when corn pith powder added (Table S1). Thus, it is possible to say that one can tune surface area, external surface area and total pores volume of the prepared zeolite. Textural properties of prepared zeolite are listed in Table S1. The TEM images of C-ZSM-5 are presented in Fig. 2A. The pores are seen on majority of the crystals in C-ZSM-5. It is expected that these intracrystalline pores and hollow structures are partially continuous and have connectivity to the micropores of the zeolite. The TEM images of ConvZSM-5 in Fig. 2B show the typical coffin-shaped morphology of MFI zeolite. Figure S3 shows the TEM images of corn stem pith powder. It was interesting to note the pith powder looks like spherical shaped particles. During the crystallization process those spherical shaped and irregular sized particles may deposit on the surface of MFI zeolite crystal. When the materials was calcined the hard template could be removed from the surface and meso-micro pores along with hallow structure have been formed. Figure S4 shows the SEM images of corn stem pith powder. The SEM image of corn stem pith powder clearly reveals that the corn pith powder looks agglomerated when we see corn pith powder alone. This agglomerated powder might be the reason for the formation of hollowness combined with lagers pores. This observation is in good agreement with nitrogen adsorption and pore size distribution. Temperature programmed desorption of ammonia (NH 3 -TPD) was performed to analyse acidic sites of prepared zeolites. The position and area of the peaks in the TPD of ammonia traces can be correlated to acidity distribution of zeolites.
The TPD traces shown in Fig. 3A indicate the presence of two desorption peaks in C-ZSM-5 and ConvZSM-5. The peak at 200 °C (low-temperature) corresponds to the very weak acidic sites whereas the broad peak at 450 °C (higher temperature) indicates the strong acid sites present in the

A B
prepared materials [27][28][29][30]. The peak position and areas under the peaks indicates that strong acid sites are more predominate in C-ZSM-5 than ConvZSM-5 zeolites. Acidity of the catalysts was calculated from the quantity of ammonia desorbed and it is found to be 0.3 mmol /g for both ConvZSM-5 and C-ZSM-5.The 27 Al MAS NMR spectra of the prepared zeolites are shown in Fig. 3B.The first resonance peak was found at 57 ppm which can be correlated to framework Al in tetrahedral coordination [31]. The second, small resonance appeared at 4 ppm is related to Al in octahedral coordination in zeolite framework. These observations indicate that almost all of the aluminium ions are incorporated in tetrahedral framework of zeolite.

Mechanism of Acidic Site Formation in the C-ZSM-5 Zeolites
It is already known that when zeolite structure contain only SiO 4 tetrahedral units it is electrically neutral and hence no meaningful acidity could be created on the surface of the zeolite. When silicon (Si) in the framework is substituted by a trivalent metal cation like Al 3+ , a negative charge can be created. This is called as isomorphous substitution. The charge can be neutralized by a proton and this proton is attached to the negatively charges oxygen connected to silicon and aluminium atoms are present as neighbours. These arrangements are called as bridging hydroxyls which are responsible for Bronsted acidity of the zeolites. In this work, we have used ammonium nitrate as a precursor to make the Bronsted acidity in C-ZSM-5 zeolites. The sodium form of C-ZSM-5 zeolites has been ion exchanged with ammonium nitrate solution, giving NH 4+ form of zeolites. When it has been calcined the ammonia was released. Eventually the H-form C-ZSM-5 zeolites are formed. Only one proton can be introduced per aluminum atom in the framework of zeolites. Strong Lewis acidity can also be created in zeolites by incorporating Lewis acidic metal cations as charge compensating species instead of protons. Further, stronger Lewis acid sites will also be created when there are defects in the zeolite framework, and unsaturated trigonal aluminium centres are generated. The total number of the Bronsted acid site in zeolite may depend on framework Si/Al ratio of zeolite precursor gel as the presence of aluminum (Al 3+ ) develop charge imbalance that could be compensated by protons, creating Bronsted acid sites. The Si/Al ratio can be varied by adding Si and Al in a particular molar proportion during zeolite synthesis and only Si-O-Si and Si-O-Al links are formed finally. Al-O-Al linkages are forbidden in zeolite because presence of two large Al atoms side by side in the place of silicon leads to an unstable structure. This is known as Lowenstein rule [32]. This phenomenon is related to decrease in the next-nearest-neighbour rule or NNN rule. The reason being, the decrease in the Al content or an increase in Si/Al ratio decreases the overall attraction between proton and the framework. The strength of the acid sites not only depends on Si/Al ratio but also on the structure of the zeolites (Fig. 4) and the formation of acidic sites in the prepared catalyst. Catalytic activity of prepared zeolites in benzylation of mesitylene with benzyl alcohol was carried out in order to confirm the presence of hierarchical micromesopores in prepared zeolites. Many researchers used benzylation of mesitylene to prove the secondary porosity generation after adding the hard templates in zeolites gel [33,34]. The expected and side products of this reaction are mentioned in Scheme 1 (See supporting information). The kinetic diameters of reactants and products in the above reactions are found to be ≈ 0.8 nm [22,23]. As we know the pore diameters of the ConvZSM-5 zeolites are smaller than this kinetic diameter, so the diffusion of these large sized reactants and products be stuck at outer surface of the zeolites crystals when micropores are present and hence conversion of reactants and selectivity of the expected products would be significantly affected. However when micro/ meso or micro/macro hierarchical porosity is present in zeolite, the said reactions may proceed favourably as diffusion of the bulky molecules is facilitated by the secondary meso/ macro-pores or hierarchical pores. Results of benzylation reaction depicted in Table 1, reveal that the benzyl alcohol conversion with C-ZSM-5 (89%) is much more than with ConvZSM-5(33%). Formation of 73% of 2-Benzyl-1, 3,5-trimethylbenzene indicates the higher selectivity of C-ZSM-5 over ConvZSM-5. The performance in this reaction evidenced the presence of hierarchical hierarchical micro-meso porosity in the prepared C-ZSM-5 zeolites. ZSM-5 possesses secondary porosity, well crystalline in nature and strong acidic sites, which are required for industrial catalytic applications.
ZhiguoYan et al. recently correlated the diffusion of organic molecule in hierarchal porous ZSM-5 zeolites in benzene-methanol alkylation reaction by molecular simulation method. It is important to note that the presence of hierarchical pores in ZSM-5 improved the anisotropic diffusion along in x-, y-and z-direction of ZSM-5 zeolite channel or pores [35]. Polyethylene oxide and urea had been effectively used to make carbon nanoparticles where it was used as hard templates to synthesis hierarchical zeolites. It exhibited two sets of pore systems. Micropores (∼0.55 nm) are from the MFI framework and mesopores (∼34.5 nm) result from hard temples of 10-40 nm in size. More importantly, the diffusion efficiency remarkably improved in hierarchical ZSM-5 zeolites. As per the results of benzylation of mesitylene with benzyl alcohol, the ZSM-5 showed better benzyl alcohol conversion of 82.0% and selectivity of desired product (2-benzyl-1,3,5-trimethylbenzene formation) has been increased considerably to 74.3% [36]. From both scientific and industrial perspectives, different hard templates have been widely employed for the synthesis of hierarchically porous zeolites during the past few decades [37]. Jiajin Huang et al. made Cu-incorporated hierarchical MEL zeolites by desilication-recrystallization method. It exhibited better selectivity in reaction of mesitylene and benzyl alcohol and also they found the catalytic performance of zeolite catalysts could be modulated by tuning the acidity and porosities of zeolites [38]. The strong acidic sites along with hierarchical pores might help for the improved benzyl alcohol conversion and selectivity in benzylation of mesitylene. In the presence of larger pores in the zeolites the diffusion of bulky molecules via pores is much easier than presence of only micropores.
Hence the reactants molecules can interact with acidic sites thereby the conversion and selectivity had been significantly improved whereas in micro porous zeolites, the bulky molecules will be hindered in the outer surfaces of catalysts and may not enter into the pores where the active site are present. Thus,the catalytic perfomances of benzylation and selectivity  was not increased over conventional ZSM-5 zeolites. The results are good agreement with present work.

Conclusion
In conclusion, the C-ZSM-5 zeolite with hierarchical micromeso porosity was prepared by a simple hydrothermal method using agricultural waste of corn plant. The synthesized C-ZSM-5 zeolites have better selectivity and catalytic performances relative to conventional micro porous ZSM-5 zeolite in benzylation of mesitylene with benzyl alcohol, which proved the presence of larger pores. The catalyst has been reused for three times and even after three times the catalytic activity (88 to 85%) and selectivity (73 to 70%) were not affected significantly in C-ZSM-5, which shows its stability nature. But the selectivity is much reduced with ConvZSM-5(32 to 22%). Table S2 shows the recycle study results of prepared Zeolites. Figures S5 and S6 shows the 1 H NMR and 13 C NMR spectra of 2-benzyl-1, 3, 5 trimethyl benzene. The structures of separated products were confirmed from the above techniques.