One-step synthesis of hierarchical [B]-ZSM-5 using cetyltrimethylammonium bromide as mesoporogen

One-step facile synthesis of boron containing ZSM-5 microspheres is developed using 1,6-diaminohexane as the structure-directing agent and cetyltrimethylammonium bromide as the mesoporogen. High boron incorporation up to Si/B ratio of 38 is achieved and evidenced by the stretching vibrations of B–O–Si at 667 cm-1 and 917 cm-1 using Fourier-transform infrared spectra. The morphology of the crystals resembles berry-like spheres with sizes approximately 15 μm, which is composed of aggregated nanocrystals having sizes around 450 nm, is observed using scanning electron microscopy. The textural properties, i.e. the surface areas and pore volumes are investigated using N2 adsorption at –196 °C. t-plot micropore volume of 0.11 cm3/g and mesopore volume of 0.14 cm3/g are obtained applying this synthesis method for mesopores having pore diameters within the range of 2–10 nm.

Weaker acidity provided by boron-containing zeolites is intriguing for reactions that would require weak/mild acidity such as double-bond isomerization of linear olefins [15], methyl tertiary butyl ether (MTBE) cracking [16], Backmann rearrangement of cyclohexanone oxime [17][18][19] and cyclohexanol dehydration reaction [20]. Mild acidity provided by a combination of Al and B sites in zeolites is especially important for methanol to olefins (MTO) [9] and isomerization of styrene oxide to phenylacetaldehyde [21]. In these reactions, strong acidity (Brønsted acid, Al-OH sites) initiates the reaction and increases the conversion; however, it also results in deposition of carbonaceous aromatics such as polymethylbenzenes and polycyclic aromatics both inside the pores and the outer surface of the crystals [22]. In order to moderate the acidity, substitution of boron in the zeolite framework is used in addition to strategies such as promoter incorporation and metal doping [23].
Owing to the weak acid sites, boron-containing zeolites enhance catalytic stability and selectivity of products by decreasing the coke formation [9,24]. In addition to boron incorporation, adding mesopores to the structure * Correspondence: bipek@metu.edu.tr This work is licensed under a Creative Commons Attribution 4.0 International License.
(hierarchical ZSM-5) was reported to increase the lifetime of the catalyst due to the improved mass transfer rate [25].
Another reason for using borosilicates is to introduce new tetrahedral site atoms into the zeolite frameworks via post-synthetic techniques. Boron incorporated zeolites are potential intermediates for these zeolites because boron atoms can be extracted under mild acidic or thermal conditions [26]. Such deboronation and new tetrahedral substitution examples include large pore Ti containing zeolites (titanosilicates) for oxidation reactions [6,27] and [Fe]-SSZ-24 for isopropylation of biphenyl [7]. Easy deboronation of boron-containing zeolites are also useful to produce interconnected mesoporosity. Deboronation using steam treatment to create mesopores in the structure of [B]-zeolite Y are reported to provide higher catalytic activity in iso-octane and 1,3,5-triisopropylbenzene (TIPB) cracking reactions [28].
ZSM-5 is a highly versatile zeolite catalyst having MFI topology. Its inherent advantages such as 10-ring interconnected channel system (straight channels along the b axis (5.3 Å * 5.6 Å) and a sinusoidal channel along the a axis (5.1 Å * 5.5 Å)) and high Si/Al molar ratios make it a promising catalyst for many applications in refinery and petrochemical processes. Therefore, synthesis of ZSM-5 containing mesopores including nano-sized ZSM-5 [29] and hierarchical ZSM-5 [30] are frequently reported in the literature with the aim of increasing the lifetime of ZSM-5 catalysts.
Nanocrystalline zeolites are known to suffer from thermodynamic instability due to high surface energy and presence of large number of surface defects. Therefore, synthesis of hierarchical and microsized crystals are more sought for combined properties such as adequate acidity and easier diffusion rates. One way of synthesizing hierarchical zeolites is to simultaneously use microporogens (micropore structure-directing agents (SDA)) and mesoporogens in the synthesis gel. Use of an ordinary cationic surfactant such as cetyltrimethylammonium bromide (CTABr) as mesoporogen has been of interest since the discovery of mesoporous MCM series materials [31] due to its low cost and commercial availability. CTABr is a quaternary ammonium surfactant, which is frequently used in the synthesis of mesoporous M41S materials, which also directs microporous ZSM-5 synthesis in transition from mesoporous MCM-41 at elevated temperatures [32]. When CTABr is simultaneously used in the presence of a structure-directing agent such as tetraalkylammonium hydroxide to create mesopores in the zeolite framework, structure direction agent and CTABr compete with each other and amorphous mesoporous materials and bulky zeolites can be formed [33]. There are some ways to prevent this phase separation. For example, subnanocrystal-type zeolite seeds having a high degree of polymerization are firstly prepared by longterm aging and then CTABr is added to the system to prepare mesoporous zeolite so competition of CTABr with SDA decreases [34,35].
Another way to prevent the competition is that SDA can work after the formation of mesoporous structure. Firstly, mesoporous structures are formed using CTABr and it is occluded in mesopores to prevent growth of zeolite crystals. Then, SDA works to form zeolite structure [36]. 1, 6-diaminohexane (HDA) is an SDA used to synthesize ZSM-5. It is weaker than other SDAs. Using HDA, Chen et al. [37] synthesized nanosized ZSM-5 to create interparticle mesoporosity. However, mesopore volume was not high enough due to the fusion of nanosized particles. Xue et al. reported successful mesopore volumes of [Al]-ZSM-5 reaching 0.46 cm 3 /g by combining HDA and CTABr as SDA and mesoporogen respectively [36]. In their method, simultaneous introduction of HDA and CTABr results in formation of mesoporous structures first, which is followed by crystallization of zeolite structures while CTABr is occluded in the mesopores to prevent growth and fusion of the zeolite nanoparticles.
In this work, mesoporous [B]-ZSM-5 zeolite synthesis was investigated using HDA as and cetyltrimethylammonium bromide (CTABr) as SDA and soft template, respectively. The effect of boron amount on crystallinity, morphology, surface area, and mesoporous volume was studied. This method enables high mesopore volume and high surface area without changing MFI topology while increased mesoporous volume and surface area are shown to be possible by increasing boron content.

Experimental procedure 2.1. Zeolite synthesis
Conventional [B]-ZSM-5 was synthesized hydrothermally for comparison following a procedure reported by Sanhoob et al. [38]. A gel mixture with the molar composition of 1 SiO 2 : 0.1 TPAOH: 0. >99 wt.%, TPABr) as SDA was added and stirred for additional 12 h. Then, the mixture was transferred into 35-mL teflon-lined autoclaves and heated at 175°C for 3 days. After that, the solid was separated by vacuum filtration, washed with distilled water several times, dried at 100°C for 12 h and calcined at 550°C using a heating rate of 1°C/min for 5 h using muffle furnace.

Characterization
Powder X-ray diffraction data were recorded on Rigaku Ultima-IV diffractometer using Cu K α source ( λ = 1.5418 Å) operated at 40 kV and 30 mA, using 2? angle range from 2°to 50°at a speed of 1°/min. Measured diffractograms were analyzed using CelRef Unit-Cell refinement software 1 , where unit cell parameters of an orthorhombic unit cell system with a space group of Pnma were refined for calcined samples.
Scanning electron microscopy (SEM) images were obtained using a QUANTA 400F Field Emission SEM with an accelerating voltage of 30 kV. The boron and silicon content of the catalyst samples were analyzed using inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer Optime 4300DV).
Surface area and pore volume of the calcined zeolites were analyzed using Micromeritics Tristar II 3020 analyzer. Samples were degassed at 300°C for 6 h under vacuum (using Micromeritics VacPrep 061) before N 2 adsorption/desorption tests were conducted at -196°C. N 2 (Oksan, 99.999%) adsorption tests were conducted at relative pressure values between 10 −5 and 0.98 following the free cell volume measurement conducted using He (Oksan, 99.999%). Micropore volumes were calculated using t-plot statistical thickness method between 3.5 Å and 5 Å with Harkins Jura thickness equation [40]. The pore size distributions were calculated using Barrett-Joyner-Halenda (BJH) Adsorption model [41] and nonlocal density functional theory (NLDFT) with cylinder pore size assumption from adsorption branch.
Fourier-transform infrared spectra (FTIR) was obtained using PerkinElmer UATR Two spectrometer equipped with an Attenuated Total Reflectance (ATR) attachment. The samples were scanned 64 times at a spectral resolution of 4 cm −1 . The spectrum was collected in the mid IR region from 500 cm −1 to 4000 cm −1 .
Thermal gravimetric analysis (TGA) of as-prepared samples were performed using a thermal analyzer (Shimadzu, DTG 60H) from 30 C to 800°C using 5°C/min heating rate and 60 cm 3 /min air flow.        [42] were additionally observed on B1-ZSM-5 and B2-ZSM-5, indicating boron incorporation to the MFI framework.

Crystallinity and Boron Incorporation
The intensity of the absorption bands at 667 cm −1 and 917 cm −1 were observed to increase with increasing boron content. The vibration at 1375 cm −1 shows presence of trigonal B species (asymmetric B-O stretching) in the framework [42], which is commonly observed when the B-containing zeolites are not fully hydrated [46]. Table 2 gives the elemental composition of the synthesized zeolites. Si/B ratios of the zeolites were obtained from ICP-EOS method following calcination of the samples. Si/B ratios of all resulting zeolites are lower than those of the synthesis gel (Si/B = 31 & 8 respectively in B1-and B2-ZSM-5 in synthesis gel). Boron content of synthesized zeolites increases with increasing boron content in the synthesis gel. Boron content in B2-ZSM-5 is found to be higher than B1-ZSM-5 as expected (Si/B of 38 vs. 78), and also evidenced from the XRD patterns ( Figure 2). Relatively high boron content of B1-ZSM-5 is beneficial for this zeolite to be further used in isomorphic substitution of boron with other tetrahedral atoms.

Samples
Si The resulting boron content of the microporous zeolite was improved by decreasing the SiO 2 :Na 2 O 3 ratio to 3.5:1 [47]. As the pH of the synthesis gel decreases with increasing H 3 BO 3 , the addition of NaOH becomes vital that accelerates the dissolution of boron source and incorporation of B into the zeolite framework [8]. Hence, the highest achieved Si/B ratio of 38 here, could be further improved by increasing the NaOH content in the synthesis gel from SiO 2 :Na 2 O 3 of 1:0.13 (or SiO 2 :Na 2 O 3 of 7.7:1) to 1:0.28.

Morphology
SEM images of the synthesized and calcined samples are shown in Figure 4. Conventional B-ZSM-5 shows typical coffin-shaped crystal morphology with particle sizes between 1 and 2 µm (Figure 4a). Mesoporous B1-ZSM-5 and B2-ZSM-5 showed different particle sizes with varying boron content. Lower boron content in the gel mixture (B1-ZSM-5) resulted in large crystals approximately 30 ×10 µm with layers observed at the crystal surface (see Figures 4b and 4c). B2-ZSM-5, having a higher boron content in the gel mixture resulted in berry-like clusters having sizes around 15 ×20 µm, composed of nanocrystals having sizes approximately 450 nm (Figures 4d and 4e). Figures 4c and 4e represent B1-ZSM-5 and B2-ZSM-5 at higher magnification.
The aggregation of nanoparticles to microspheres were reported when HDA was used as the SDA with [36] or without the presence of CTABr [37]. As Xue et al. reported, presence of CTABr is known to increase  and explained by enhanced nucleation and growth rates in presence of higher Si content [36]. This enhanced crystallization rates disrupted the comparable rates of mesopore formation and zeolite crystallization , which led to lower mesopore volumes (see Table 3).

Textural property characterizations
TGA of as-prepared B1-ZSM-5 was performed prior to any textural analysis via N 2 adsorption to make sure total removal of the organics used in synthesis. Based on the TGA and the differential TGA results ( Figure   5), the calcination temperature was selected as 580°C, which was maintained for 10 h for complete combustion of the organics. Figure 6 represents the N 2 adsorption-desorption isotherms obtained at -196°C following the calcination of the samples at 580°C. According to Figure 6, addition of CTABr resulted in increased N 2 adsorption capacity and hysteresis. Conventional B-ZSM-5 exhibited a typical type-I isotherm according to IUPAC classification, reflecting traditional zeolite having only micropores. However, B1-ZSM-5 and B2-ZSM-5 showed type-IV isotherms with a significant hysteresis loop in the P/P 0 range from 0.45 to 0.9, indicating mesoporosity. Increasing amount of boron content increased the adsorption capacity and hysteresis. The hysteresis type can be designated as H2(b), indicating pore-blocking with a wider range of pore necks [48]. Table 3 gives the textural characterization of the synthesized zeolites using N 2 adsorption isotherms. The microporous surface area was calculated using t-plot method, whereas BET surface area was calculated assuming multilayer adsorption model. The external surface area was calculated as the difference between the BET surface area and the microporous surface area. B2-ZSM-5, with the highest boron incorporation into the framework resulted in larger external surface areas and larger S EXT / S BET ratios when compared to B1-ZSM-5 and conventional B-ZSM-5.
The micropore volumes of mesoporous B-ZSM-5 were calculated using statistical thickness method (tplot) for all samples, which showed slight decreases compared to conventional B-ZSM-5 (micropore volume of B1-ZSM-5 and B2-ZSM-5 are 0.10 and 0.11 cm 3 /g, respectively, see Table 3). The mesopore volume values were  calculated by subtracting the t-plot micropore volume from the single point pore volume obtained at P/P 0 = 0.98. As it can be inferred from Table 3, increasing amount of boron content increased the mesopore volume of the synthesized zeolites. The highest mesopore volume was obtained on B2-ZSM-5, which was 0.14 cm 3 /g. B1-ZSM-5 showed a mesopore volume of 0.11 cm 3 /g.
[Al]-ZSM-5 resulted in 0.32 cm 3 /g of mesopore volume, whereas B2-ZSM-5 resulted in 0.14 cm 3 /g, which could be due to lower interaction of boron content with CTABr when compared to Al sites.

Formation mechanism
We investigated the formation mechanism of the microspheres of mesoporous B2-ZSM-5 using XRD, SEM, and N 2 adsorption analysis of calcined samples at the 7th, 10th and 14th day of synthesis (the synthesis of B2-ZSM-5 is repeated for this part). At the 7th day of synthesis, the peaks characteristic of MFI phase (with the most observable peaks at 8.02°and 8.94°belonging to the (101) and (020) planes of Pnma space group) appeared at low intensity values, indicating the starting of the crystallization (see Figure 8). In addition to the MFI peaks, a broad peak at 2 θ value of 2.92°corresponding to a d-spacing of 3 nm was observed together with a broad peak between 20°and 25°indicating a mesoporous structure. At the 10th day of synthesis, the peak at 2.92°shifted to 2.72°(d-spacing of 3.2 nm), and the characteristic MFI peaks gained more intensity. At the 14th day of synthesis, the peak at 2.72°and the broad peak between 20°and 25°disappeared as the MFI peaks reached their maximum intensity. When the intensity of the peak belonging to the (101) plane of ZSM-5 (Pnma space group) observed at 7.94°was compared at the 10th day and 14th day of the synthesis, it can be said that the sample at the 10th day has 68% crystallinity with respect to the one at the 14th day.
SEM images (see Figures 9a-9c) show that aggregates of spherical particles having sizes approximately 200 nm (see Figure 9a) occur at the 7th day of the synthesis that are mesoporous (see Table 4 for mesopore volume and Figure 10) as also indicated from XRD pattern. The micropore formation was negligible (0.01 cm 3 /g) when compared to mesopore volume found by N 2 adsorption at -196°C (0.54 cm 3 /g, Table 4). The pore size distribution of this sample showed a fairly narrow pore size range with a maximum value of 2.9 nm (see Figure 11), which is in agreement with the d-spacing value found in XRD analysis. The morphology and the narrow pore size distribution at around 3 nm found for this mesoporous sample resemble those of an MCM-41 structure [1].   This mesoporous spherical structure enlarges to diameters of~13-15 µm (see Figure 9b) while some part of it converts into microporous zeolite at the 10th day of the synthesis. The micropore volume obtained at the 10th day is 0.09 cm 3 /g (see Table 4) while the mesopore volume value dropped to 0.30 cm 3 /g. Then, further crystallization under the effect of HDA gives more defined crystals at the 14th day (see Figure 9c) resulting in a higher micropore volume (see Table 4, 0.13 cm 3 /g) and a slightly lower mesopore volume (0.14 cm 3 /g).
The trends in the micropore-mesopore volume changes during the synthesis confirm a sequential mesostructure formation and crystallization of mesoporous structure to zeolite. Xue et al. also reported formation of mesoporous materials (nanoparticles similar to Al-MCM-41) in the presence of CTABr, followed by conversion of the mesostructured material to the crystalline ZSM-5 starting from the 9th day of synthesis using XRD, FTIR, and N 2 adsorption [36]. They explained this sequential mechanism by relatively weak interaction of the structure-directing agent: HDA with the oligomers, which prevented the competition between CTABr and SDA (and hence prevented the formation of a mixture of amorphous mesostructure and zeolite crystals) [36]. A similar mechanism is suggested here for mesoporous B2-ZSM-5 with slight changes in the resulting mesoporous structure observed at the 7th day of the synthesis. The mesopore volumes of B2-ZSM-5 are found to be smaller (0.54 cm 3 / g) than mesoporous Al-ZSM-5 reported by Xue et al. (0.89 cm 3 /g mesopore volume at the 7th day of [Al]-ZSM-5 [36]). The reason for this can be the lower B concentration incorporated into the framework unlike high Al content of Al-ZSM-5. The weaker interaction of B and CTABr could result in slightly smaller mesopore volumes. Furthermore, the hysteresis observed in Figure 10 for the 7th day synthesis can be classified as the H2 type [48], which is attributed to blocked pores unlike type H1, attributed to open pores typical for MCM-41. Nevertheless, formation of aggregated spherical mesoporous particles of sizes approximately 200 nm at the beginning of the synthesis is still suggested here.
The pore diameter distribution in B2-ZSM-5 was also investigated at the 7th, 10 th , and 14th day of the hydrothermal synthesis. The pore sizes get wider from~3 nm to~5-10 nm as the conversion from total mesopore structure to small zeolite crystal aggregates takes place (see Figure 11). The pore size of 2.9 nm and 3.3 nm observed at the 7th and 10th day of synthesis are in very good agreement with the d-spacing values of 3 and 3.2 nm calculated from XRD ( Figure 8). A similar pore size distribution is also reported for mesoporous Al-ZSM-5 by Xue et al. The pore diameter of the mesostructures was observed approximately 3 nm, whereas with progressed crystallization, pore size distribution becomes wider and shifts to larger pore diameters~12 nm [36].

Conclusion
One-step synthesis of hierarchical borosilicate [B]-ZSM-5 has been shown for Si/B ratios ranging from 38 to 78 using 1,6-diaminohexane as the structure-directing agent and cetyltrimethylammonium bromide as the mesoporogen. The morphology of the samples resembled microspheres with particle sizes varying between 10 and 30 µm formed by aggregation of nanoparticle crystals. The microspherical form was related to the used structure agent, i.e. 1,6-diaminohexane, whereas the nanoparticles having sizes approximately 450 nm were related to the presence of cetyltrimethylammonium bromide acting as the crystal growth inhibitor. Mesopore volumes reaching 0.14 cm 3 /g was related to the interaction of boron with cetyltrimethylammonium bromide due to the increasing behavior of mesopore volumes with increasing boron content. The formation mechanism of the microspheres is shown to start with amorphous and mesoporous particles, which then crystallizes into ZSM-5 with the effect of 1,6-diaminohexane.