The Effect of Alkaline Treatment to the Structure of ZSM 5 Zeolites

Hierarchical zeolites combining microand mesoporosity were prepared using desilication method in alkaline solution of NaOH 0.2 M on two types of ZSM5: ”template free” commercial ZSM5 (Si/Al 8.62) and ‘templated’ as-synthesized ZSM5 (Si/Al 25). The powder X-ray diffraction patterns revealed that crystallinity and short-range order in the alkaline treated zeolites were virtually unchanged compared to both of the parent zeolites. The surface measurement on the ”template free” ZSM5 zeolites showed that the surface area was reduced by 2.33%, but the adsorption isotherm can be categorized into Type IV which is typical of for mesoporous material, supported by the change in mesopore volume, and the BJH pore size distribution (from 10 to 18 nm). On the other hand, the surface area of the alkaline treated assynthesized ZSM5 was increased by 8.25%, but its isotherm adsorption curve falls into Type I for microporous structure. The mesopore volume was increased by 26%, from 0.037 cm/g to 0.046 cm/g, with the intrinsic zeolite properties were mainly preserved. Based on these results, it can be concluded that the existence of organic template plays an important role in preserving the zeolitic structure during the alkaline treatment.


Introduction
Catalytic cracking, alkylation, and isomerization are well-established processes in industry, making use of the unique advantages of zeolites, such as their high thermal stability, high selectivity through homogeneous pore shape and size, and acidity [1].However, the microporous nature of these solid acids imposes diffusion limitations in reactions involving bulky hydrocarbons.Mass-transfer constraints limit the catalytic activity and occasionally also the selectivity and lifetime.In such cases, it is convenient to increase the accessibility and molecular transport to/from the active sites in zeolites to reach their full catalytic potential.Hierarchical zeolites have emerged as an important class of materials leading to improved catalytic performance compared to their microporous parents [2].This is typically attributed, to the shortened micropore diffusion path length as a result of a secondary mesopore network of an inter-or intracrystalline nature [3].
Recently, alkaline treatment (desilication) was explored as a post-synthesis tool to increase the porosity, and thereby the activity, of zeolites.The selective extraction of framework silicon by treatment in alkaline solutions, referred to as desilication or base leaching, is now a widely used top-down method to prepare hierarchical porous zeolites [4][5][6][7][8].Desilication was firstly applied to study chemical changes of MFI crystals upon contact with NaOH solutions [9][10].The desilication process was further improved by Groen et al. [5], who determined that the best results for ZSM5 are obtained with 0.2 M NaOH for 30 min at 328 K with a Si/Al ratio between 25 and 50.But, the fast kinetics of silicon dissolution in desilication sometimes causes limited opportunities to precisely control the process of mesopore formation.A recent strategy to tailor the degree of mesoporosity is NaOH treatment of partially detemplated zeolites [6].Several studies show that templates can be helpful for successful alkaline treatment.Pérez-Ramírez et al. [7] reported that partial detemplation followed by alkaline treatment can be used to tune the desilication process and that the addition of template molecules (TPA + or TBA + ) during desilication can be used as pore growth moderators.The templatecontaining regions in the crystal are less prone to desilication than template-free regions [7,12].Accordingly, the template content could determine the volume of attackable zeolite by NaOH and the amount of extra-mesoporosity.
Herein we report a synthesis method to tailor the degree of mesoporosity in hierarchical zeolites, consisting of the combination of partial detemplation and silicon extraction.In this study alkaline treatment was applied to commercially available "template free" ZSM5 (Si/Al 8.62) and 'templated' as-synthesized ZSM5 (Si/Al 25).The change in surface area properties and crystallographic structure are discussed in order to obtain the information on the effect of organic template to the success of adding mesoporosity in the ZSM5 structure.The results of this work could be useful in improving the synthesis of mesoporous zeolite and its further application.
Synthesis of templated ZSM5.ZSM5 zeolite was synthesized following the procedure reported by Wang et al. [13] with some modification.The gel of ZSM5 zeolite was prepared from a homogeneous mixture with molar composition of: 7.84 (TPA) 2 O : Al 2 O 3 : 50 SiO2: 2301.74H 2 O.The mixture was stirring and aged at 373K for 3 h.After, the mixture was transferred into an autoclave for further crystallization at 423K for 144 h.The product was collected by filtration and air dried in order to preserve the organic template.
Alkali-treatment procedure on "template free" ZSM5.ZSM5 zeolite used in this study, with a SiO 2 /Al2O 3 molar ratio of 39.4, was supplied by Zeolyst in Na form, and is hereafter referred to as 'untreated' sample.The Alkaline-treatment used is based on the procedure reported by Ogura et al. [4].First, 375 ml of aqueous 0.2 M NaOH solution was heated to 338K in the flask connected to a reflux condenser using a water bath.Then, 5 g of ZSM5 was slowly transferred into the heated solution, and the solution stirred at that temperature for 30 min.Then slurry was cooled down immediately using an ice bath and filtered using 0.7 µm pore size cellulose-mixed ester filter paper.The filtered cake was dried in an air oven at 383 K overnight.The result was analyzed to determine the concentrations of Si and Al after alkali-treatment.The alkali-treated ZSM5 is abbreviated as ZSM5-cAT.
Alkali treatment on 'templated' as-synthesized ZSM5.Alkali treatment of as-synthesized 'templated' zeolite was performed using the same procedure as employed with the commercial zeolite.The result was then calcined under similar treatment conditions to the zeolite parent commercial and then calcined at 823 K for 5 h to remove the template.The alkali-treated 'templated' ZSM5 was labeled as ZSM5-sAT.
Characterization of ZSM5 zeolitesThe XRD patterns were obtained using a Phillips PW 1710 diffractometer using Cu Kα radiation.The nitrogen adsorption and desorption isotherms at 77 K were measured using a Quantachrome Quadrawin Version 3.12.The samples were outgassed for 10 h at 300 °C before being measured.The surface area was calculated by using the Brunauer-Emmett-Teller (BET) method based on the adsorption data in the partial pressure (P/P 0 ) range 0.05-0.35.The pore-size distribution was determined by using the Barrett-Joyner-Halenda (BJH) adsorption model.Micropore volume was obtained by t-plot analysis.Total pore volume was obtained from the amount of nitrogen adsorbed at P/P 0 = ca.0.99.Mesopore volume was calculated by subtracting micropore volume from total volume.Scanning electron microscopy experiments and ratio of Si/Al were performed with JEOL JSM-6390 electron microscopes.Thermogravimetric analysis was performed on a Mettler Toledo Star System.Functional groups of zeolite were analyzed by Shimadzu IR Prestige-21.

Results and Discussion
Alkali-treatment on the "template free" ZSM5.The XRD pattern of the untreated zeolite exhibits the MFI structure as the only crystalline phase (Fig. 1) as shown in IZA Zeolite Atlas [14].The x-ray diffraction analysis confirms the preservation of the long-range crystal ordering in the alkaline treated zeolite, but peak broadening and lower peak intensity were also observed.This can be explained by the occurrence of hydroxide anion from the NaOH solution attacking the silanol groups and Si-O-Si or Si-O-Al bonds thus causing damage to the crystal structure.
Figure 2 shows the N 2 adsorption and desorption isotherms at 77 K for the parent commercial ZSM5 (ZSM5-c) and ZSM5-cAT.The parent zeolite material exhibits a type I isotherm with high uptaking at low relative pressures, confirming its microporous character [15].Alkaline treatment of ZSM5 zeolite leads to an isotherm representing both types of I and IV behavior, with a remarkably enhanced uptake of nitrogen at higher pressures accompanied by a hysteresis loop [15].This suggests the presence of both micro and mesoporosity as reported by van Laak et al. [12] and Wang et al. [13].
The BJH pore-size model also confirms the presence of mesoporosity in the alkaline-treated material, showing a pronounced development of a broad pore-size distribution centered around 10-20 nm from the original 1-2 nm (Figure 3).
In Table 1, the textural properties of the samples are listed.The values of S-meso and V-meso of parent zeolite were significantly lower compared to alkaline treated zeolite.A further increase in the mesopore volume area is observed from 0.3803 m 2 /g for the parent to 0.4509 m 2 /g for ZSM5-cAT.Similar trend was also reported by van Laak et al. [12] in study of alkaline treatment of H-ZSM-5.A likely explanation for these observations is that with alkaline treatment layers of the external surface area of the crystallites are dissolved, thereby increasing the mesoporosity of the agglomerates.The Si/Al ratio (Table 2) is changed upon the alkaline-treatment, due to significant removal of Si from the framework.However, this desilication process seemed not to damage the structure globally as shown by similar XRD pattern, compared to parent commercial ZSM5 (Figure 1).
Scanning electron microscope images of parent and alkaline-treated ZSM5 are shown in Figure 4.The effects of treatment on the morphology of the crystals cannot be seen clearly from the picture.Nevertheless, it can be seen that before the alkaline treatment, spherelike ZSM5 particles are separated from each other, whereas after the alkaline treatment, the particles appear to agglomerate.However, the effect of the alkalinetreatment on the outer surface of the zeolite is not visible.
Alkali treatment on as-synthesized 'templated' ZSM5, called ZSM5-s, serves as an approach to combine mesoporosity with intact microporosity in the structure.The parameter considered in this synthesis is the molar ratio of zeolite ZSM5 which is related to the number of Al atoms in the zeolite framework.
According to a study by Groen et al. [5], the alkali treatment results in optimal formation of mesopores with the ratio of Si/Al in the range of 25-50.
XRD pattern of the zeolite product exhibits the MFI structure as the only crystalline phase (Figure 5).The difractogram shows no significant difference from the sharpness of the peaks of ZSM 5 samples before and after alkali treatment.The existence of peaks appeared at almost the same angles (2θ) indicating that the crystallinity of the zeolite before and after treatment are similar to each other.However, there are changes in the relative intensity of the peaks of the resulting zeolite.
Relative intensity is a parameter indicating the amount or number of the measured crystal planes.
Linear isotherms of adsorption-desorption of nitrogen for ZSM-5 before and after alkaline treatment (ZSM5-s and ZSM5-sAT, respectively) are shown in Figure 6.
The nitrogen adsorption isotherm of ZSM5-s is of type I with a plateau at higher relative pressures and no distinct hysteresis loop.The ZSM5-sAT sample also shows a similar nitrogen uptake, indicative of unchanged pore properties with respect to the parent ZSM5-s.Correspondingly, characteristics of pore size and pore size distribution of both materials were also observed from the graph data using the BJH measurement.Similarity in pore size distribution of ZSM5-s and ZSM5-sAT is clearly seen in Figure 7. ZSM5 parent and ZSM5-sAT have a majority distribution at 1.7 nm.
Based on Figure 7, it can be concluded that the distribution of pores of ZSM5 parent and ZSM5-sAT are dominant in the micropore region.On the other hand, differences in physicochemical properties between ZSM5 parent and ZSM5-sAT are observed, as can be seen in Table 3.
External surface area was changed with respect to the parent sample from 349 m 2 /g to 378 m 2 /g and the mesopore volume area increased by 26% from 0.0368 cc/g to 0.0464 cc/g.And for all products, micropore surface areas and volumes were also increased from those in the parent ZSM5-s.
Figures 8 and 9 show that after treatment, the morphology of ZSM5 zeolite changed.It was obvious that from the comparison between parent ZSM5-s and alkaline treated ZSM5-sAT, some cracks and faults appeared on the surface of ZSM5 particles after alkalitreatment.While the SiO 2 /Al 2 O 3 molar ratio in ZSM5   after alkali-treatment is unchanged as summarized in Table 4.
The SiO 2 /Al 2 O 3 ratio slightly decreased after 30 minutes of alkali-treatment.This is in contrast with the amount of Si eluted in 30 min which corresponds to 39.97% of Si in the framework of the commercial ZSM5 after treatment (ZSM5-cAT).It can be said that in the presence of tetraalkylammonium cations as template, the zeolite surface is better shielded from the attack by the hydroxyl group (OH -) with respect to sodium cations, i.e. the higher affinity of TPA + for the zeolite combined with its intrinsic steric hindrance, slows down the kinetics of the desilication process.
In support of this behavior, the presence of TPA + cations is also responsible for the formation of smaller mesopores compared to treatment in the absence of the template.Treatment with template also creates differences in the size of intra-crystalline pores.As shown by the BJH pore size distribution (Fig. 7), the alkaline treated ZSM5-sAT shows the formation of small pores (1.7 nm) compared to ZSM5-cAT, the alkali treated zeolite without template (10-18 nm).This can be explained by the fact that the protected regions inhibit extensive silicon extraction.Thus, the zeolite still preserves the micropore structure.Because of that, more extensive experimental conditions are suggested to be applied in order to generate the formation of larger pores in the presence of the template.Longer treatment times and/or higher temperatures are required to increase substantial size pore intracystalline in ZSM5-sAT.This result is rather different to that reported by van Laak et al. [12].In their work, mesoporous ZSM5 was still obtained although the increase of mesoporosity was also smaller than the results from ZSM5 without template as starting material.
From Table 4, we can also see that during the alkali treatment of templated zeolites, ZSM5-sAT, aluminum species was slowly dissolved in relation with the acid sites of zeolites.This process created smaller crystallites and inter-crystalline mesoporosity.The presence of the template inside the micropores assures that the acid sites were not affected by the treatment regardless of the framework type and Si/Al ratio.This environment enables the decoupling of accessibility and acidity, and can be used to improve the scope as well as the understanding of zeolite catalyzed reactions.

Conclusions
Alkaline treatment on a commercial ZSM5 and assynthesized ZSM5 "with template" to provide additional mesoporous systems into zeolite crystals have been carried out.After treatment, the commercial ZSM5 showed a graph of type IV adsorption isotherms with a hysteresis loop that is characteristic of the mesoporous material and an increase in V meso of 18.56%.The BJH method showed an increase in distribution of pores in the mesoporous region, of 10-18 nm.On the other hand, although ZSM5 synthesis "with template" was also subjected to alkaline treatment, there were no significant changes in pore character, in which microporosity is still dominant.However, the results of BET analysis shows that after treatment there was an increase in surface area and mesopore volume.To conclude, the presence of organic template inside the pore of zeolite structure influenced the creation of mesoporosity through alkaline treatment.

Table 3 . Chemical Composition and Textural Properties of Parent ZSM5-s and Alkaline-Treated ZSM5-sAT
a.BET method, b.t-plot method, c.V meso =V ads p/po=0.99-V micro