Altering Dendrimer Structure of Fibrous-Silica-HZSM5 for Enhanced

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Altering Dendrimer Structure of Fibrous-Silica-HZSM5 for Enhanced Product Selectivity of Benzene Methylation Aishah Abdul Jalil,*,†,‡ Afifah Syafiqah Zolkifli,† Sugeng Triwahyono,§ Anis Farhana Abdul Rahman,† Nik Norhanani Mohd Ghani,† Muhamed Yusuf Shahul Hamid,† Fatin Hazira Mustapha,† Siti Maryam Izan,§ Bahador Nabgan,† and Adnan Ripin†,‡ †

School of Chemical and Energy Engineering, Faculty of Engineering, ‡Centre of Hydrogen Energy, Institute of Future Energy, and School of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia

Downloaded via UNIV OF NEW ENGLAND on September 30, 2018 at 01:34:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

§

S Supporting Information *

ABSTRACT: Dendrimeric fibrous silica HZSM-5 (HFSZ) catalysts were prepared using a microemulsion technique and ZSM-5-seeding crystallization with different oil phases, benzene, toluene, and xylene, and denoted as FB, FT, and FX, respectively. The HFSZ catalysts possessed a well ordered crystalline structure with chestnut-like spherical particles, rich with dendrimeric fiber morphology. It was found that the hydrophobicity of the oil phase significantly affected the physicochemical properties of the HFSZ catalysts that altered their activity toward benzene methylation. The density of dendrimeric fibers and ensuing acidities were found to play crucial roles in enhancing the selective production of toluene and p-xylene via transalkylation and dealkylation of bulkier aromatics. The production of ethylbenzene (EB) using commercial HZSM-5 could be suppressed to give major products of toluene (84%) and p-xylene (58%) when using FB and FX at 673 and 623 K, respectively.

1. INTRODUCTION Toluene and p-xylene are widely used in industry as intermediates for commodity petrochemicals and valuable fine chemicals, for example as solvents in dilution, extraction, pharmaceuticals, paint stripping, machinery, insecticides, and rubber manufacturing and also as additives in fuel.1 However, the production of these chemicals by traditional catalytic reforming and naphtha pyrolysis is no longer convenient due to the shortage of petroleum resources.2 In recent years, benzene alkylation has been investigated as an alternative route for the synthesis of both chemicals from natural gas and coal.3 Benzene alkylation over solid acid catalysts has become more popular than conventional alkylation using acidic ionic liquid catalysts due to its advantages such as high stability, strong acid sites, large pores, and economic practicality (complete conversion of benzene can be achieved with 95− 100% selectivity in alkyl benzene).4,5 Solid acid catalysts can be tungstated and sulfated zirconia, heteropolyacids, metal complexes, sulfated tin oxide, silica, and silica−alumina; however, they have several shortcomings, including easier formation of volatile compounds, instability, and increasing the risk of water poisoning, high cost, and weak acidity leading to lower catalytic activity.6 Consequent attempts to use zeolites, including ZSM-11, ZSM-5, MCM-22, ITQ-2, mordenite, β(BEA), Y-zeolite, SAPO-34, and SAPO-5/MnAPO-5, showed some improvement in benzene alkylation due to shape selectivity resulting from a high surface area, good thermal © XXXX American Chemical Society

stability, and greener materials compared with homogeneous catalysts.7 ZSM-5 is a popular heterogeneous catalyst that has been used at an industrial level since 1980 for various applications including xylene isomerization, aromatic alkylation, conversion of methanol to gasoline, and decomposition of natural oils.8 Its unique structure, tunable acidity, and high thermal stability compared with other zeolites offer high shape selectivity and improved catalytic activity.9 However, its microporous structure has drawbacks including low active site accessibility and diffusion limitations, leading to rapid deactivation and low conversion and product yield.10 For example, an HZSM-5@ silicate-1 core−shell composite has been used in alkylation that showed high selectivity but a low reaction rate due to diffusion limitation through a thick silicalite-1 layer.11 Thus, modification of the ZSM-5 structure is crucial to enhance catalytic activity. Hybridization with silica in superior structural topographies consisting of microporosity and mesoporosity is one option.12 The high accessibility and low diffusion limitation of fibrous Special Issue: 2018 International Conference of Chemical Engineering & Industrial Biotechnology Received: July 11, 2018 Revised: September 7, 2018 Accepted: September 11, 2018

A

DOI: 10.1021/acs.iecr.8b03147 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

morphology of the sample was identified by field emission scanning electron microscopy−energy dispersive X-ray (FESEM-EDX) and transmission electron microscopy (TEM) using JEOL JSM-6701F and JEOL JEM-2100F microscopes, respectively. The textural properties were determined from nitrogen physisorption at 77 K using a Beckman Coulter SA 3100 surface area analyzer. The functional groups of the catalysts were identified using PerkinElmer Spectrum GX Fourier-transform infrared (FTIR) spectrometer. The surface defect and oxygen vacancy were confirmed using JEOL JESFA100 ESR spectrometer. The analysis regarding the optical absorption properties of the catalysts were obtained using a PerkinElmer UV−vis/DRS spectrophotometer in the range of 200−800 nm. 2.4. Catalytic ActiviTy Evaluation. Benzene methylation was carried out under atmospheric pressure at a temperature range of 423−673 K in a microcatalytic pulse reactor equipped with an online sampling valve for gas chromatographic analysis. Initially, 0.2 g of catalyst were activated under air (Fair = 80 mL/min) for 1 h followed by hydrogen (Fhydrogen = 100 mL/ min) for 1 h. When the temperature was stable, a benzene− methanol mixture with molar ratio of 1:1 was fed into the reactor at a weight hourly space velocity (WHSV) of 2.0 h−1 and 80 mL/min H2 as carrier gas flow rate. The products were trapped at 77 K before flushing out to the gas chromatograph. The reaction products were analyzed by online 7820A Agilent Gas Chromatograph equipped with a flame ionization dectector (FID) and HP-5 column. Conversion of benzene, selectivity of toluene, ethylbenzene, p-xylene, o-xylene, cracking products, and yield were calculated as follows

silica nanoparticles (KCC-1) discovered by Polshettiwar et al.,13 led to the successful synthesis of fibrous silica HZSM-5 (HFSZ) with a high surface area and abundant strong acid sites, showing high catalytic activity toward cumene hydrocracking and carbon monoxide methanation.10,14 The unusual urchin-like structure of HFSZ, a ZSM-5 core covered with abundant dendrimeric silica fibers, means it is a versatile catalyst for various reactions. Hypothetically, a size-controllable HFSZ would enhance the accessibility of various compounds and would have great potential in the petroleum and petrochemical industries. To the best of our knowledge, there has been no research into this for HFSZ. The microemulsion technique is a reliable approach for controlling the properties of materials including their morphology, particle size, and surface area and is dependent on the surfactants, solvents, and cosolvents used during synthesis.15,16 Herein, we report for the first time the effects of different oil phases (benzene, toluene, and xylene) on the size of HFSZ. The differences in hydrophobicity and hydrophilicity of these oils is hypothesized to vary the physicochemical properties of HFSZ. The catalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM)/ transmission electron microscopy (TEM), N2 adsorption− desorption, Fourier transform infrared (FTIR), electron spin resonance (ESR), and ultraviolet−visible (UV−vis) spectroscopy. The catalytic activity of HFSZ was assessed by benzene methylation, and the structure of the catalysts and mechanism of benzene methylation were also proposed. The ability to control the physicochemical properties of catalysts would be useful for related catalyst design in various applications.

2. EXPERIMENTAL SECTION 2.1. Materials. Cetyltrimethylammonium bromide (CTAB), urea, 1-butanol (C4H9OH), benzene (C6H6), toluene (C7H8), xylene (C8H10), tetraethyl orthosilicate (TEOS), and commercial ZSM5 (Si/Al = 23) were purchased from Merck Sdn. Bhd., Malaysia, while ammonium nitrate (NH4NO3) was obtained from QRec, Malaysia. Deionized water was used for the preparation of all reagents. 2.2. Catalyst Preparation. In this study, a microemulsion system coupled with zeolite crystal seeds (CBV2314, Zeolyst International) was used to prepare fibrous ZSM-5 (FSZ). A 1 mol portion of tetraethyl orthosilicate (TEOS) was dissolved in a mixture of 28 mol of toluene and 1.62 mol of 1-butanol. Then, the ZSM-5 seed was added to the solution and stirred for 30 min. A mixed solution of 0.9 mol of urea, 0.27 mol of CTAB, and distilled water was then added. The resulting solution was exposed to intermittent microwave radiation (400 W) for 6 h. The solid product was isolated by centrifugation at 20 000 rpm, followed by washing with acetone and distilled water and was then dried overnight in air at 373 K. Finally, the product was calcined at 823 K for 6 h under an air atmosphere to obtain the FSZ. Protonated fibrous silica ZSM5 (HFSZ) was obtained by 2fold protonation of FSZ with NH4NO3 accompanied by drying at 383 K for overnight and calcination at 823 K for 3 h under an air atmosphere. The catalysts prepared under oil phase of benzene, toluene, and xylene were denoted as FB, FT, and FX, respectively, while the bare commercial ZSM-5 was denoted as HC. 2.3. Catalyst Characterization. The crystallinities of the catalysts were confirmed by X-ray diffraction (XRD) recorded on a D8 ADVANCE Bruker X-ray diffractometer. The surface

XB =

benzene feed − benzene product × 100% benzene feed n(C toluene) × 100% n(total product)

Stoluene =

Sethylbenzene =

Sp ‐ xylene =

So ‐ xylene = Sc1 − c6 = Y=

n(Cethylbenzene) n(total product) n(Cp ‐ xylene)

n(total product)

n(Co ‐ xylene) n(total product)

× 100%

× 100%

× 100%

n(Cc1 − c6) × 100% n(total product)

(1)

(2)

(3)

(4)

(5)

(6)

XBSproduct 100

(7)

where X is conversion, n is number of moles, C is concentration, S is selectivity, and Y is yield of the corresponding compound.

3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Studies. Figure 1 shows wide-angle XRD patterns of HFSZ catalysts prepared in different oil phases (benzene, toluene, and xylene), that are denoted as FB, FT, and FX, respectively. A series of characteristic peaks was observed in the range 2θ = 2−30 at B


Article


Figure 1. XRD diffractogram of HC and HFSZ catalysts.

Figure 2. FESEM images of catalysts (A) FB, (B) FT, (C) FX. (D) TEM image of FT and particle size distribution of all catalysts based on FESEM images (inset).

7.8° (101), 8.8° (103), 23.0° (112), 23.3° (200), 23.9° (105), and 30.0° (224), indicating the typical ZSM-5 structure (JCPDS file no. 01-086-1157).17 The peak intensities of the HFSZ catalysts are lower compared to those of the commercial HZSM-5 (HC), suggesting a significant loss in the crystallinity of catalysts due to the formation of abundant dendrimeric silica fibers at the outer surface of the ZSM-5 core.10 The peak intensities were of the order FB < FT < FX, and there were no significant changes in peak position. This may be due to the different hydrophobicity and hydrophilicity of the oils affecting the formation of micelles during the preparation process, subsequently altering the crystallinity of the silicazeolite catalysts (Table 1). A similar decline in crystallinity was reported in the recrystallization of HZSM-5 via surfactantmediated reassembly.18 Morphological studies by FESEM (Figure 2A−C) show that the particles size of the HFSZ catalysts is in the following order: FB < FT < FX. By measuring the projected areas of 100 individual particles in the FESEM images, the average size distributions of the spherical and well-ordered HFSZ particles could be determined: 300−500, 400−600, and 600−1000 nm for FB, FT, and FX, respectively (Figure 2A inset).19 A TEM image of FT (Figure 2D) confirmed the chestnut-like spherical shape of the particle, rich with dendrimeric fibers on its outer surface. This is similar to the structure of KCC-1.13 This unusual configuration is believed to benefit benzene methylation by controlling the diffusion limitation of the reactant and/or products and enhancing the catalytic performance. 3.2. Textural Properties of Catalysts. Figure 3 illustrates the N2 adsorption−desorption isotherms and pore size distribution for all catalysts. In accordance with the IUPAC

classification, the HFSZ catalysts clearly displayed type IV isotherms with H3 hysteresis loops, signifying a mesoporous material with nonuniform slit-shaped pores.10 The FB and FT catalysts showed higher N2 uptake compared with FX, indicating their higher porosities. Two-step capillary condensation was observed, where the first step is at a partial pressure (P/P0) of 0.3, and the second step at a higher P/P0 of 0.9, attributing to the intra- and interparticles textural porosity of the catalysts. The interparticles textural porosity of the HFSZ catalysts decreases in the following order: FB > FT > FX. This shows an inverse correlation with the crystallinity shown in Table 1. HC exhibited a typical isotherm for microporous materials. It was shown that FX possesses a similar isotherm to HC, suggesting a higher microporosity than mesoporosity compared with the other HFSZ catalysts. This is in agreement with the pore distributions derived from the (Nonlocal density functional theory) NLDFT method shown in Figure 3, in which the FB and FT exhibited similar ranges of major sharp peaks at 3−6 nm and small peaks at 20−50 nm. The intensity of these peaks decreased significantly for FX, demonstrating its lower mesoporosity. A decrease in surface area and pore volume from FB to FX was also observed (Table 1). Mesopores can be defined as the interdendrimer distance,10 and therefore, a decrease in interparticles pores could represent a decrease in the density of dendrimeric fibers. It is also supposed that the bigger the HFSZ particle size, the shorter the silica fibers, which could explain the increased peak intensities or more pronounced ZSM-5 peaks in the XRD diffractogram of FX. A similar result was previously reported in

Table 1. Physico-chemical Properties of Catalysts catalyst

SiO2/Al2O3 ratioa

crystallinity [%]b

surface area [m2/g]c

total pore volume [mL/g]c

VMic [mL/g]c

VMeso [mL/g]d

pore width [nm]e

BAS [au]f

LAS [au]f

FB FT FX HC

88 80 52 23

28 30 40 100

717 691 307 346

1.31 0.988 0.394 0.203

0.0218 0.0177 0.124 0.140

1.29 0.970 0.270 0.0634

4.41 4.40 1.27

0.0429 0.0067 0.0181 0.2740

0.0350 0.0176 0.0309 0.1620

a Obtained by microwave plasma atomic emission spectroscopy (MPAES). bThe ratio of crystallinity was calculated based on the highest XRD peak at 23.9°. cObtain by BJH method. dMesopore volume was calculated by subtracting micropore volumes from total pore volumes estimated from the amount of N2 adsorbed at P/P0 = 0.9814. eObtain from the DFT method. fThe area intensity of Brönsted and Lewis acid sites in the pyridine peak was calculated using the deconvolution method. The catalysts were outgassed at 423 K.

C


Article


Figure 3. Nitrogen adsorption/desorption isotherm and NLDFT pore-size distribution of all catalysts.

Figure 4. (A) FTIR KBr of all catalysts. (B) Summary of their bands intensities. (C) Evacuated FTIR. (D) Summary of their bands intensities.

the development of micro- and mesostructured zeolite crystals by adjusting the amount of silica.20 3.3. Vibrational Spectroscopy. All HFSZ catalysts were subjected to FTIR spectroscopy, and the spectra in the 1400− 400 cm−1 region are shown in Figure 4A. All catalysts showed five main bands at 1084, 964, 794, 540, and 450 cm−1, corresponding to Si−O−T asymmetric stretching, external Si− OH groups, Si−O−T symmetric stretching, two fivemembered rings of the MFI-type ZSM-5 structure, and Si− O−T bending vibrations, respectively.21 All these adsorption bands are typical of MFI-type zeolites. Almost all of the HFSZ bands increased when changing the oil phase from benzene to toluene to xylene, indicating an improvement in their structure completeness as in agreement with the XRD data. The two latter bands (540 and 450 cm−1) were in contrast with the HC bands, further confirming the presence of silica fibers surrounding the ZSM-5 seed. From the band intensities summarized in Figure 4B, it is clear that FX possesses the highest number of almost all bands, particularly for the Si−O− T asymmetric and bending modes, demonstrating its wellformed structure compared with the other HFSZ catalysts. However, the external −OH bands (at 964 cm−1) decreased in the following order: FB > FT > FX, further evidencing the excess formation of Si−O−T bonds in the framework of the catalysts during preparation and/or the calcination process.22

To investigate the hydroxyl groups, the HFSZ catalysts were evacuated at 673 K for 1 h prior to FTIR analysis to remove the physisorbed water. Figure 4C shows the IR spectra in the 3800−3400 cm−1 region. Using Gaussian curve fitting in this region, bands were deconvoluted (Figure S1). The spectra of the HFSZ catalysts consisted of five main bands (3740, 3660, 3610, 3700, and 3520 cm−1), which were assigned to nonacidic terminal silanol groups located on the external surface of the zeolite, extra-framework Al−OH (EFAl) species, Brønsted OH bands associated with bridging hydroxyl groups (Si(OH)Al) located inside the zeolite structure, and perturbed OH groups in the surrounding lattice defects or extra-lattice oxygen,23 respectively. For clarity, the intensities of these bands are summarized in Figure 4D. It was noted that FB and FT were rich with terminal and perturbed silanol groups, further verifying their higher density and/or longer dendrimeric fibers compared with FX. In addition, the numbers of EFAl groups were also higher, confirming the additional generation of the extra-framework in FB and FT. This may also explain the lower intensity of the main aluminosilicate framework in the XRD pattern compared with FX. Furthermore, HC possessed a significantly higher number of Brønsted OH groups compared with the HFSZ catalysts, indicating that the formation of silica dendrimeric fibers had an advantage in weakening the acidity D


Article


Figure 5. IR spectra of 2,6-lutidine adsorbed on all catalysts at heating in a vacuum at (a) 298, (b) 323, (c) 348, (d) 373, (e) 398, (f) 423, (g) 448, and (h) 473 K.

Figure 6. (A) ESR and (B) UV-DRS spectra of HFSZ catalysts. (C) Relationship between the ESR and UV-DRS peaks intensities.

evidenced by the higher intensity of the bands at 3700 and 3520 cm−1 as well as the Si(OH)Al groups at 3610 cm−1 (Figure 4D). The excess exclusion of EFAl (at band 3660 cm−1 in Figure 4D) from the framework during the preparation of FB and/or calcination may also contribute to the higher number of Lewis acid sites compared to FX.27 Figure S2 shows the deconvoluted IR spectra of all catalysts by Gaussion curve fitting adsorbed on Brønsted and Lewis acid sites outgassed at 473 K. The total amounts of both acid sites are shown in Table 1. The results confirmed the acidic strength of the catalysts in the following order: HC > FB > FX > FT. It is possible that the more developed the aluminosilicate framework of the HFSZ catalysts (Figure 4B), the sparser their dendrimeric fibers (Figure 4D), and the weaker their acidic sites (Figure 5). The use of a selective oil phase is important for the appropriate generation of acidic sites in the HFSZ catalysts. The different hydrophobicity and hydrophilicity of the oil phases affected the bulk of the aluminosilicate framework and the formation of the dendrimeric fibers of the HFSZ catalysts that altered their acidic centers. 3.5. Oxygen Vacancies, Lattice Defects, and Skeletal Coordination Studies. ESR spectroscopy was used to detect the existence of oxygen vacancies (OVs). Figure 6A shows the ESR signal of the HFSZ catalysts, and the peak intensities at each g-value are summarized in Figure 6C. The signals at g = 1.94, 1.98, and 2.00 are ascribed to the paramagnetic OV, trapped or unpaired electrons localized in OVs, and protonic

of the catalysts. This may be beneficial in the later acidcatalyzed reaction.24 3.4. Acidity Strength of HFSZ Catalysts. The acidic sites in the HFSZ catalysts were qualitatively probed using 2,6lutidine adsorption monitored by IR spectroscopy. 2,6Lutidine (pKb = 7.4) is more basic than pyridine (pKb = 8.8) and is used to study the relatively weak Brønsted acid sites and the Lewis acid sites.25 Figure 5 shows the IR spectra of adsorbed 2,6-lutidine for all catalysts in the 1750−1300 cm−1 region as a function of outgassing temperature at 298−473 K with 25 K increments. The catalysts were expected to show doublet bands, which are clearly observed for HC at 1681 and 1623 cm−1 and 1446 and 1338 cm−1, corresponding to Brønsted and Lewis acid sites, respectively.26 In comparison with HC, both acid sites in the HFSZ catalysts were reduced, particularly the Lewis acid sites, this may be due to the high density of dendrimeric fibers hindering the accessibility of the 2,6-lutidine probe molecule to the acidic sites.27 In addition, both acidic sites decreased with elevated outgassing temperature, indicating their weak acidity, predominantly in FT. Once the catalysts had been outgassed at 473 K, the remaining acidic sites were considered to be strongly acidic. The number of Brønsted (at bands 1639 and 1562 cm−1) and Lewis (1457 and 1365 cm−1) acid sites for FB were higher compared to FX. This may be due to the higher density of dendrimeric fibers in FB than in FX, and thus more Brønsted acid sites were generated from the abundant perturbed OH groups. This is E


Article


Figure 7. Proposed mechanism for the formation of HFSZ catalysts.

framework to interact with the positively charged CTAB hydrophilic groups. Then, addition of a silica source (TEOS) which is more hydrophobic, leads to the construction of a silica network to form a core−shell-like structure, where the core is ZSM-5 and the shell consists of dendrimeric silica fibers. Calcination removes the surfactant, cosurfactant, and oil, leaving the dendrimeric fibrous ZSM-5-SiO2 or so-called FSZ catalysts. Hydrothermal treatment using NH4NO3 solution then protonates the catalyst to form HFSZ. According to the FTIR-KBr and XRD data, the degree of order of aluminosilicate and/or silica framework increased as follows: FB < FT < FX. This led to an improvement in crystallinity. Similarly, the FESEM results showed an increasing trend with crystallite size when changing the oil phase from benzene to toluene to xylene, and this also correlated with the decrease in specific surface area. These results may explain why the number of −OH groups decreased with increasing formation of the Si−O−T skeletal aluminosilicate framework, also affecting the number of OVs, defect sites, and acidity of the catalysts. Comparing the FB with FX, the higher the density of dendrimeric fibers, the higher the total numbers of Brønsted and Lewis acid sites were generated, and this also be the reason for the stronger acidity of the former than the latter. Conversely, the measured SiO2:Al2O3 ratio was reduced by half on changing the oil phase from benzene to xylene (Table 1). This may be due to the higher amount of EFAl possessed by FB, increasing the difficulty of dealumination by nitric acid compared with the generally tetrahedrally coordinated FX.32 In this study, it seems that the higher the hydrophobicity of the oil phases, the larger the boundary of the aqueous phase of the reverse micelle that led to the formation of larger zeolite core size and sparser silica fiber of the HFSZ. Thus, it can be dictated that the physicochemical properties of the studied HFSZ catalysts can be controlled by the hydrophobicity and hydrophilicity of the oil phase used. 3.7. Catalytic Performance and Proposed Reaction Pathway. The catalytic activity of the HFSZ catalysts was assessed during benzene methylation at a temperature of 573− 673 K in a microcatalytic pulse reactor under nitrogen carrier gas. Detailed results on conversion and products distribution

acid sites near the oxygen atoms on the surface of the catalyst as hydrogen-bonded OH groups, respectively.27,28 Figure 6B shows the UV diffuse reflectance spectra of HFSZ catalysts at 200−400 nm, and the band intensities are summarized in Figure 6C. All catalysts exhibited main adsorption bands at 230 and 330 nm, corresponding to skeletal tetrahedral and octahedral (EFAl) coordination in the zeolite structure.29 It was observed that FB possessed the greatest amount of octahedral coordination out of all the HFSZ catalysts. This further explains the low intensity of the main aluminosilicate framework in the XRD diffractogram, and the high EFAl Al− OH bands in the evacuated FTIR spectra. With regard to the skeletal tetrahedral coordination, the three ESR signals generally increased with the size of the catalyst’s aluminosilicate framework from FB to FT. This is most probably due to the higher density of dendrimeric silica fibers and Si−O−Si network. Si−O bonds break during calcination in an oxygendeficient atmosphere leading to greater formation of OVs and structural defects.30 Additionally, it has been previously reported that the presence of dendrimeric morphology may contribute to OVs in the catalyst.14 The minor decrease in ESR signals for FX is in accordance with the lowest amount of perturbed OH groups at 3700 cm−1 (Figure 4D). This further supports the finding that the sparser the dendrimeric silica fibers, the lower the loss of extra-lattice oxygen and lattice defects compared with the other HFSZ catalysts. It may also be possible to achieve complete tetrahedral coordination in the aluminosilicate framework of the HFSZ catalyst using toluene and xylene, with greater hydrophobicity, rather than benzene. This controllable number of OVs and defect sites contributes to the different degrees of acidity of each catalyst and would be beneficial in the benzene methylation process.31 3.6. Proposed Mechanism of HFSZ under Different Types of Oil Phases. Based on the results above, a mechanism for the formation of an HFSZ catalyst is proposed in Figure 7. The mixing of CTAB, urea, water, toluene, and 1butanol, as a surfactant, amine source, hydrophilic solvent, oil phase solvent, and cosurfactant, respectively, led to the formation of reverse micelles.10 Subsequent addition of the hydrophilic ZSM-5 seed allowed the negatively charged ZSM-5 F


Article


Figure 8. (left) Effect of reaction temperature on yield of products when using (A) HC, (B) FB, (C) FT, (D) FX. (right) Relationship between acidity and pore structure of the catalysts.

acidity, as possessed by FT, is sufficient to initiate the production of toluene, xylene isomers, and/or bulkier aromatics. However, the number of meso- and micropores is extremely important to control the product distribution. The fact that there is a 103−104 times faster distribution of p-xylene than both m- and o-xylene, and reduced diffusion limitation from the dendrimeric fibers of HFSZ, may explain the selective production of p-xylene and toluene in this study.37,38 Based on the trend of product distribution (Figure 8A−D) and the relationship between the acidity and pore structure of the catalysts (Figure 8E), the benzene methylation pathway using the HFSZ catalysts is proposed in Figure 9. First, the

are listed in Table S1. Figure 8A−D show the overall yields of the products, and the relationship between acidity and pore structure is shown in Figure 8E. Generally, it clearly observed from Figure 8A that the production of ethylbenzene (EB) and others by commercial HC at 623 K can be suppressed to give major products of toluene and/or p-xylene when using HFSZ catalysts (Figure 8B−D).33 As confirmed by a gas chromatograph, the other products supposed to consist of straight aliphatic (C5−C8) and higher alkyl-aromatic compounds (C9− C10) such as trimethylbenzene, methylethyl-benzene, diethylbenzene, and tetramethylbenzene. This is in agreement with the products of several similar reactions reported in the literature.34 It seem that the C5−C8 were produced at lower to moderate temperature (573−623 K), prior to toluene and pxylene becoming the major products at higher temperatures, particularly when using FB. While, the C9−C10 were the major products of others for FX at 623−673 K. Hence, these results verify that multiple parallel and consecutive reactions of bulkier aromatics take place to give the final major products of toluene and p-xylene under the reaction conditions studied.35 In comparison with HFSZ, HC was found to produce surplus aliphatic rather than alkyl-aromatic compounds (data not shown), which is likely due to its higher acidity, and only molecules like toluene, EB, and p-xylene that have diameters of 0.67 nm can penetrate the system, unlike bulkier alkyl-aromatic compounds.34 However, the strong acidity of HC allowed further alkylation to occurr outside the zeolite channel to produce the C9−C10 aromatics at 623−673 K. In the case of FB, the number of cracked products and bulkier products increased with temperature and, at 673 K, all yields decreased to increase toluene production to 84%. The stronger acidities and availability of space may induce the high selectivity of the final desired product. Although the selectivity has to be improved, the highest yield of p-xylene (58.4%) was achieved at 623 K when using FT, which may be due to its balanced acidities as well as micro- and mesopore numbers that led to a high production of p-xylene compared with the other catalysts.36 Further increasing the temperature up to 673 K led to production of bulkier alkyl-aromatic compounds, implying a significant role of the acidity and topology of the catalysts in the reaction. The low average pore width of FX (Table 1) and its sparser dendrimeric fibers that provide stronger acidity, decrease the diffusion limitation and allow further alkylation for production of bulkier aromatics outside the fibers. It can therefore be suggested that relatively weak

Figure 9. Proposed reaction pathway for benzene methylation over all catalysts: (A) disproportionation, (B) transalkylation, (C) dealkylation.

methanol adsorbed onto the Brønsted acid sites of the catalyst reacts with benzene to form toluene.39,40 The toluene then further reacted with methanol to form mostly p-xylene, and subsequent consecutive disproportionation gave trimethylbenzenes and tetramethylbenzenes. The toluene was mainly produced by FB, while the p-xylene was produced by FX. The EB was formed when benzene reacted with ethylene as a consequence of dimerization of methanol.41 Further methylation of EB led to the formation of methylethylbenzenes, but this route seems favored when using HC, most probably due to the high mesopore volume that offered less diffusion resistance and allowed the formation of bulkier products. Subsequently, transalkylation of trimethylbenzenes and tetramethylbenzenes occurred to give p-xylene, while sequential dealkylation of bulkier methylethylbenzenes also took place to reduce the products to toluene, which may occur when using FB and FT.35 In summary, compared with HC, the density of dendrimeric fibers and the acidities of G


Article

Industrial & Engineering Chemistry Research Table 2. Comparative Studies of Benzene Methylation surface area (m2/g)

BAS (au)

LAS (au)

B/L

HC FB FT FX

346 717 691 307

0.0429 0.0067 0.0181 0.2740

0.0350 0.0176 0.0309 0.1620

1.23 0.381 0.586 1.69

ZSM-5 ZSM-5 ZSM-5 Na/ZSM-5 Cu/ZSM-5 Co/ZSM-5 MgO/ZSM-5 Pd/ZSM-5 ZnO/ZSM-5 Pt/ZSM-5 Co3O4−La2O3/ZSM-5

409 408

0.1388 1.0 2.8 1.5

6.54 4.20 0.25 0.40

0.0318

1.38 6.34 0.50

catalyst

4.2 0.7 0.6 345 404 367

0.0159

conv of benzene (%) yield of toluene (%)

259

yield of p-xylene (%)

ref

84.2a 94.9b 61.5b 93.3a

12.0a 83.9b 46.6b 6.22a

17.8a 5.83b 12.8b 58.4a

this this this this

12.5 51.0 44.3 47.9 46.5 46.4 55.5 51.5 52.0 37.5 51.5

8.13 24.5 23.1 22.5 24.9 24.1 28.3 25.8 24.4 26.3 29.6

2.20 17.9 12.0 14.3 13.7 14.9 21.1 18.4 18.2 8.25 16.7

40 43 42 42 42 42 43 43 43 43 44

study study study study

a

Reaction at 623 K. bReaction at 673 K.

HFSZ played crucial roles in enhancing the selective production of toluene and p-xylene in this study. Comparisons with previous studies on ZSM-5-based catalysts are shown in Table 2. These show that the HFSZ catalysts perform well in the selective production of toluene and p-xylene.

■

4. CONCLUSION HFSZ catalysts were successfully prepared in different oil phases (benzene, toluene, or xylene) by combining the microemulsion technique and zeolite seed crystal methods. Different degrees of hydrophobicity of the oil phases resulted in different sizes, surface areas, mesoporosities, dendrimeric silica fiber densities, numbers of OVs, defect sites, and acidities of the HFSZ catalysts that altered their activity during benzene methylation. The characterization data showed that the higher the hydrophobicity of the oil, the better the arrangement of the aluminosilicate framework in the catalysts, with a larger HFSZ core and bigger size of HFSZ catalyst (FB < FT < FX). In comparison with HC, the formation of dendrimeric silica fibers on HFSZ catalysts may weaken the acidity of the catalyst; however, the bigger the particle size of the catalyst, the shorter their silica fibers, leading to lowered acidity due to the decrease in formation of the extra-framework of alumina. The formation of OVs and defects was also inversely proportional to the increase in dendrimeric silica fibers. The density of dendrimeric fibers and ensuing acidities were found to play essential roles in enhancing the selective production of toluene and p-xylene via transalkylation and dealkylation of bulkier aromatics. The production of EB by HC can be suppressed to give major products of toluene (83.9%) and p-xylene (58.4%) when using FB and FX at 673 and 623 K, respectively. This study has shown that the HFSZ catalysts, compared with several previous ZSM-5-based catalysts, are promising for selective production of toluene and p-xylene via benzene methylation.

■

Gaussian curve-fitting of FTIR spectra in evacuated system of all catalysts (Figure S1); Gaussian curve-fitting of IR spectra of 2,6-lutidine adsorbed at 473 K on all catalysts for Bronsted and Lewis acid sites (Figure S2); and product distribution for benzene methylation over each catalyst (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: 60-7-5535581. Fax: 60-7-5536165. E-mail: aishahaj@ utm.my (A.J.A.). ORCID

Aishah Abdul Jalil: 0000-0003-0811-3168 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the financial support by the Research University Grant from Universiti Teknologi Malaysia (Grant No. 17H46), Fundamental Research Grant Scheme (Grant No. FRGS/1/2017/STG07/UTM/01/1 (4F969)) and the award of MyMaster Scholarship (A.S.Z.) from Ministry of Higher Education, Malaysia.

■

REFERENCES

(1) Li, Y.; Xue, B.; Yang, Y. Synthesis of ethylbenzene by alkylation of benzene with diethyl oxalate over HZSM-5. Fuel Process. Technol. 2009, 90, 1220−1225. (2) Hu, H.; Zhang, Q.; Cen, J.; Li, X. High suppression of the formation of ethylbenzene in benzene alkylation with methanol over ZSM-5 catalyst modified by platinum. Catal. Commun. 2014, 57, 129−133. (3) Laredo, G. C.; Merino, P. M. V.; Hernandez, P. S. Light cycle oil upgrading to high quality fuels and petrochemicals: A Review. Ind. Eng. Chem. Res. 2018, 57, 7315−7321. (4) Sani, Y. M.; Daud, W. M. A. W.; Aziz, A. A. Activity of solid acid catalysts for biodiesel production: a critical review. Appl. Catal., A 2014, 470, 140−161. (5) Galadima, A.; Muraza, O. Role of zeolite catalysts for benzene removal from gasoline via alkylation: A review. Microporous Mesoporous Mater. 2015, 213, 169−180.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03147. H


Article

Industrial & Engineering Chemistry Research (6) Triwahyono, S.; Jalil, A. A.; Mukti, R. R.; Musthofa, M.; Razali, N. A. M.; Aziz, M. A. A. Hydrogen spillover behavior of Zn/HZSM-5 showing catalytically active protonic acid sites in the isomerization of n-pentane. Appl. Catal., A 2011, 407, 91−99. (7) Zhang, Y. F.; Liu, X. L.; Sun, L. Y.; Xu, Q. H.; Wang, X. J.; Gong, Y. J. Catalytic cracking of n-hexane over HEU-1 zeolite for selective propylene production: Optimizing the SiO2/Al2O3 ratio by in-situ synthesis. Fuel Process. Technol. 2016, 153, 163−172. (8) Wang, Y. R.; Liu, M.; Zhang, A. F.; Zuo, Y.; Ding, F. S.; Chang, Y.; Song, C. S.; Guo, X. W. Methanol usage in toluene methylation over Pt modified ZSM-5 catalyst: Effects of total pressure and carrier gas. Ind. Eng. Chem. Res. 2017, 56, 4709−4717. (9) Naskar, M. K.; Kundu, D.; Chatterjee, M. Synthesis of ZSM-5 Zeolite Particles Using Triethanol Amine as Structure-Directing Agent. J. Am. Ceram. Soc. 2012, 95, 449−452. (10) Firmansyah, M. L.; Jalil, A. A.; Triwahyono, S.; Hamdan, H.; Salleh, M. M.; Ahmad, W. F. W.; Kadja, G. T. M. Synthesis and characterization of fibrous silica ZSM-5 for cumene hydrocracking. Catal. Sci. Technol. 2016, 6, 5178−5182. (11) Nishiyama, N.; Miyamoto, M.; Egashira, Y.; Ueyama, K. Zeolitemembrane on catalyst particles for selective formation of pxylene in the disproportionation of toluene. Chem. Commun. 2001, 18, 1746−1747. (12) Teh, L. P.; Triwahyono, S.; Jalil, A. A.; Mukti, R. R.; Aziz, M. A. A.; Shishido, T. Mesoporous ZSM5 having both intrinsic acidic and basic sites for cracking and methanation. Chem. Eng. J. 2015, 270, 196−204. (13) Polshettiwar, V.; Cha, D.; Zhang, X.; Basset, J. M. High-surfacearea Silica Nanosphere (KCC-1) With a Fibrous Morphology. Angew. Chem., Int. Ed. 2010, 49, 9652−9656. (14) Teh, L. P.; Triwahyono, S.; Jalil, A. A.; Firmansyah, M. L.; Mamat, C. R.; Majid, Z. A. Fibrous silica mesoporous ZSM-5 for carbon monoxide methanation. Appl. Catal., A 2016, 523, 200−208. (15) Pogrzeba, T.; Schmidt, M.; Milojevic, N.; Urban, C.; Illner, M.; Repke, J. U.; Schomacker, R. Understanding the role of non-ionic surfactants during catalysis in microemulsion systems on the example of rhodium-catalyzed hydroformylation. Ind. Eng. Chem. Res. 2017, 56, 9934−9941. (16) Moon, D. S.; Lee, J. K. Tunable synthesis of hierarchical mesoporous silica nanoparticles with radial wrinkle structure. Langmuir 2012, 28, 12341−12347. (17) Treacy, M. M. J.; Higgins, J. B. Collection of simulated XRD powder patterns for zeolites, 4th revised ed.; Elsevier, 2001; pp 203− 204. (18) Wang, X.; Zhang, J.; Zhang, T.; Xiao, H.; Song, F.; Han, Y.; Tan, Y. Mesoporous ZnZSM-5 zeolites synthesized by one-step desilication and reassembly: a durable catalyst for methanol aromatization. RSC Adv. 2016, 6, 23428−23437. (19) Sidik, S. M.; Triwahyono, S.; Jalil, A. A.; Aziz, M. A. A.; Fatah, N. A. A.; Teh, L. P. Tailoring the properties of electrolyzed Ni/ mesostructured silica nanoparticles (MSN) via different Ni-loading methods for CO2 reforming of CH4. J. CO2 Util. 2016, 13, 71−80. (20) Hu, H.; Lyu, J.; Rui, J.; Cen, J.; Zhang, Q.; Wang, Q.; Han, W.; Li, X. The effect of Si/Al ratio on the catalytic performance of hierarchical porous ZSM-5 for catalyzing benzene alkylation with methanol. Catal. Sci. Technol. 2016, 6, 2647−2652. (21) Rahman, A. F. A.; Jalil, A. A.; Triwahyono, S.; Ripin, A.; Aziz, F. F. A.; Fatah, N. A. A.; Jaafar, N. F.; Hitam, C. N. C.; Salleh, N. F. M.; Hassan, N. S. Strategies for introducing titania onto mesostructured silica nanoparticles targeting enhanced photocatalytic activity of visible-light-responsive Ti-MSN catalysts. J. Cleaner Prod. 2017, 143, 948−959. (22) Salleh, N. F. M.; Jalil, A. A.; Triwahyono, S.; Ripin, A.; Sidik, S. M.; Fatah, N. A. A.; Salamun, N.; Jaafar, N. F.; Hassim, M. H. New direct consecutive formation of spinel phase in (Fe,Co,Ni)Al2O4 composites for enhanced Pd(II) ions removal. J. Alloys Compd. 2017, 727, 744−756.

(23) Sidik, S. M.; Jalil, A. A.; Triwahyono, S.; Abdullah, T. A. T.; Ripin, A. CO2 reforming of CH4 over Ni/mesostructured silica nanoparticles (Ni/MSN). RSC Adv. 2015, 5, 37405−37414. (24) Corma, A. Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. Rev. 1995, 95, 559−614. (25) Fatah, N. A. A.; Triwahyono, S.; Jalil, A. A.; Ahmad, A.; Abdullah, T. A. T. n-Heptane isomerization over mesostructured silica nanoparticles (MSN): Dissociative-adsorption of molecular hydrogen on Pt and Mo sites. Appl. Catal., A 2016, 516, 135−143. (26) Annuar, N. H. R.; Triwahyono, S.; Jalil, A. A.; Basar, N.; Abdullah, T. A. T.; Ahmad, A. Effect of Cr2O3 loading on the properties and cracking activity of Pt/Cr2O3-ZrO2. Appl. Catal., A 2017, 541, 77−86. (27) Setiabudi, H. D.; Jalil, A. A.; Triwahyono, S. Ir/Pt-HZSM5 for n-pentane isomerization: Effect of iridium loading on the properties and catalytic activity. J. Catal. 2012, 294, 128−135. (28) Ningthoujam, R. S.; Lahiri, D.; Sudarsan, V.; Poswal, H. K.; Kulshreshtha, S. K.; Sharma, S. M.; Bhushan, B.; Sastry, M. D. Nature of Vn+ ions in SnO2: EPR and photoluminescence studies. Mater. Res. Bull. 2007, 42, 1293−1300. (29) de Freitas Silva, T.; Dias, J. A. C.; Maciel, C. G.; Assaf, J. M. Ni/ Al2O3 catalysts: effects of the promoters Ce, La and Zr on the methane steam and oxidative reforming reactions. Catal. Sci. Technol. 2013, 3, 635−643. (30) Strunk, J.; Vining, W. C.; Bell, A. T. A Study of Oxygen Vacancy Formation and Annihilation in Submonolayer Coverages of TiO2 Dispersed on MCM-48. J. Phys. Chem. C 2010, 114, 16937− 16945. (31) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape Selective Catalysis in Industrial Applications, 2nd ed.; Marcel Dekker: New York, 1996. (32) Jin, Y.; Zhang, L.; Liu, J.; Zhang, S.; Sun, S.; Asaoka, S.; Fujimoto, K. Mesopore modification of beta zeolites by sequential alkali and acid treatments: Composition-dependent T-atoms removal behaviour back donating to hierarchical structure and catalytic activity in benzene alkylation. Microporous Mesoporous Mater. 2017, 248, 7− 17. (33) Rahmani, E.; Rahmani, M. Alkylation of benzene over Fe-based metal organic frameworks (MOFs) at low temperature condition. Microporous Mesoporous Mater. 2017, 249, 118−127. (34) Ali, S. A.; Ogunronbi, K. E.; Al-Khattaf, S. S. Kinetics of dealkylation−transalkylation of C9 alkyl-aromatics over zeolites of different structures. Chem. Eng. Res. Des. 2013, 91, 2601−2616. (35) Ahn, J. H.; Kolvenbach, R.; Al-Khattaf, S. S.; Jentys, A.; Lercher, J. A. Methanol Usage in Toluene Methylation with Medium and Large Pore Zeolites. ACS Catal. 2013, 3, 817−825. (36) Fatah, N. A. A.; Triwahyono, S.; Jalil, A. A.; Salamun, N.; Mamat, C. R.; Majid, Z. A. n-Heptane isomerization over molybdenum supported on bicontinuous concentric lamellar silica KCC-1: Influence of phosphorus and optimization using response surface methodology (RSM). Chem. Eng. J. 2017, 314, 650−659. (37) Mirth, G.; Cejka, J.; Lercher, J. A. Transport and Isomerization of Xylenes over HZSM-5 Zeolites. J. Catal. 1993, 139, 24−33. (38) Tsai, T. C.; Liu, S. B.; Wang, I. Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts. Appl. Catal., A 1999, 181, 355−398. (39) Vos, A. M.; Schoonheydt, R. A.; Proft, F. D.; Geerlings, P. Reactivity descriptors and rate constants for acid zeolite catalyzed ethylation and isopropylation of benzene. J. Phys. Chem. B 2003, 107, 2001−2008. (40) Van der Mynsbrugge, J.; Visur, M.; Olsbye, U.; Beato, P.; Bjørgen, M.; Van Speybroeck, S. V.; Svelle, S. Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts. J. Catal. 2012, 292, 201−212. (41) Odedairo, T.; Al-Khattaf, S. S. Comparative study of zeolite catalysed alkylation of benzene different chain length: H-ZSM-5 versus mordenite. Catal. Today 2013, 204, 73−84. I


Article

Industrial & Engineering Chemistry Research (42) Adebajo, M. O.; Howe, R. F.; Long, M. A. Methylation of benzene with methanol over zeolite catalysts in a low pressure flow reactor. Catal. Today 2000, 63, 471−478. (43) Hu, H.; Lyu, J.; Cen, J.; Zhang, Q.; Wang, Q.; Han, W.; Rui, J.; Li, X. Promoting effects of MgO and Pd modification on the catalytic performance of hierarchical porous ZSM-5 for catalyzing benzene alkylation with methanol. RSC Adv. 2015, 5, 63044−63049. (44) Gao, K.; Li, S.; Wang, L.; Wang, W. Study of the alkylation of benzene with methanol for the selective formation of toluene and xylene over Co3O4−La2O3/ZSM-5. RSC Adv. 2015, 5, 45098−45105.

J


Altering Dendrimer Structure of Fibrous-Silica-HZSM5 for Enhanced

Recommend Documents