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2.1.1. MaterialsTetraethyl orthosilicate (TEOS, 99%) and tetrapropylammoniumhydroxide [TPAOH, 25% (by mass) aqueous solution] were obtainedfrom Meyer Chemical, China. Ethanol (90%), aluminum tri-tertbutoxide (ATTB, 99%) and ammonium hydroxide (28%–30%) so-lutions were purchased from Shanghai Lingfeng Chemicals, whilehexadecyltrimethoxysilane (HTS) was obtained from Gelest, USA.Distilled H2O was used throughout this work. All commercialchemicals were used directly without further purifications.2.1.2. SynthesisHierarchical ZSM-5 (denoted as ZSM-5-HTS-x, x is themolar ratio ofSi/Al) was prepared via a modified approach developed by the author[44]. In a typical synthesis of ZSM-5-HTS, 3.12 g TEOS, 0.21 g HTS and0.035 g ATTBwere dissolved in 20.0ml ethanol under stirring. Thismix-ture was further stirred for 30 min to form a solution, followed by theaddition of another solution of 2.0 g TPAOH in 5.0 g ethanol under vig-orous stirring; hydrolysis occurred at this stage as evidenced by heat re-lease, although a stable so-called “clear solution” is produced (thetransparent “clear solution” is stable up to one month in a closed vial).The “clear solution” was stirred for 1 h before being transferred to apetri dish where the solvent was allowed to evaporate overnight. Atransparent dry gel resulting from solvent evaporation was ground,then moved to a teflon cup. The cup was placed in a teflon-lined auto-clave of 150 ml, and 10.0 g of water is added outside the cup to createsteam for the hydrothermal synthesis conditions. The autoclave wasmoved into an oven set at 180 °C and kept there for 3 days for asteam-assisted crystallization. The final solid, retaining the shape ofthe original dry gel particles, was filtrated and washed with water,and then calcined at 550 °C for 20 h by a ramp of 3 °C•min−1in air.To prepare a conventional ZSM-5 with the same Si/Al ratio, 3.24 gTEOS, 0.07 g ATTB, 2.0 g TPAOH and 7.7 ml ammonium hydroxidewere mixed in 30.0 g
DI-H2O and stirred for 4 h, then transferred toan autoclave and allowed to crystallize for 72 h at 180 °C [44].Combus-tion to remove the template was conducted using the same procedureas for ZSM-5-HTS. For both samples, the Si/Almolar ratiowas controlledto be 20, 40, 70 and 180.2.2. CharacterizationsX-ray diffraction patterns of powders were carried out on a RigakuD/max 2550 VB/PC (40 kV, 100 mA) X-ray diffractometer operatedwith a monochromatized Cu Kα radiation (λ = 0.15406 nm) enabledby nickel filtration of Cu Kβ. The scan rate was 8(°)•min−1, rangingfrom 5–50°. N2 physisorption was employed to analyze the surfacearea and pore architecture of both powders. The specific surface areasof both conventional ZSM-5 and HPZ ZSM-5-HTS were deduced fromthe Langmuirmethod and the Brunauer–Emmett–Teller (BET) method,respectively, for the purpose of comparison by using a MicromeriticsASAP 2020 Accelerated Surface Area and Porosimetry System. A micro-pore volume is calculated from the standard t-plot approach, while thetotal volume pores were based on the adsorption capacity at a relativepressure of 0.99. The macro- plus meso-pore volume was referred toas the subtraction of themicropore volume fromthe total pore volume.Mesopore size distribution was calculated from a Barrett–Joyner–Halenda (BJH) model using the adsorption branch of an isotherm.Scanning electronic micrographic (SEM) images were taken from aHitachi S-4800(II) field-emission SEM apparatus (FESEM). Ammoniatemperature programmed desorption (NH3-TPD) was measured usinga PX200A type TPD/TPR instrument from Tianjin Pengxiang Tech.Corp., with 0.10 g conventional ZSM-5 and 0.25 g ZSM-5-HTS sampledrespectively. Before measurement, each sample was heated under ultrapure He atmosphere at a flowrate of 30 ml•min−1at 500 °C for 1 hto remove surface adsorbate, cooled to 150 °C and saturated with 5%NH3–He before flushing with He to remove physisorbed NH3.Theeffluent of NH3-TPD was quantified by a flame ionization detector(FID) under a heating rate of 10 °C•min−1. Fourier transform infrared(FT-IR) spectra of all samples were recorded on a Nicolet 6700 FTIRspectrometer in KBr pellets.2.3. Catalytic performance assessmentCatalytic reaction was carried out at 0.1MPa of pressurewith a 2.5 gcatalyst, which was squeezed into strips with sizes of 1–2 mm, in astainless steel reaction tube of 10 mm ID and 600 mm length. Theflow rate of N2 which was used as carrier gas was 50 ml•min−1. Pre-mixed solutions ofmethanol and benzenewith amole ratio 1:1were in-troduced directly into the reactor at the rate of about 0.20 ml•min−1without being preheated. All the reactions were carried out at 460 °C.Reaction products were analyzed by gas chromatography (Agilent6820). The selectivity for reaction products S, the yield of reaction prod-ucts Y,andtheconversionofbenzene X are defined asXB ¼ NM 100% ð1ÞSTX ¼ P þ QN 100% ð2ÞYTX ¼ P þ QM 100% ð3ÞST ¼ PN 100% ð4ÞSX ¼ QN 100% ð5Þwhere M is the mole number of benzene in the raw material, N is themole number of the benzene series in the products, P is the mole num-ber of toluene in the products and Q isthemolenumberofxyleneintheproducts. B, T, and X stand for benzene, toluene, xylene respectively, andTX is the sum of benzene and toluene.3. Results and Discussions3.1. SEM images of dry gel and XRD patterns for ZSM-5Details of preparation to a ZSM-5 with a Si/Al ratio of 70 has beenreported in a preceding report by one of the present authors, the struc-tural characterizations of this specific sample were also included [44].Itis noteworthy that themethodmerits an improved pore connectivity asevidenced by 129Xe NMR measurements, and is thereby superior toalternative routes. Herein, we show the synthesis and characterizationresults for the prepared ZSM-5 with different Si/Al ratios to validatethe wide applicability of the modified method. Representative SEMimages of the dry gel after ethanol evaporation for a Si/Al ratio of 180are disclosed in Fig. 1. At lowmagnification, silica gel made up of aggre-gated particles in the sample can be observed unanimously, showingthat the dry gel is uniform in morphology [Fig. 1(a)]. When we zoomin, a more detailed structure is exhibited in Fig. 1(b); the gel is foundto consist of large numbers of tiny particulates ranging around 20 to40 nmin diameter.