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MFI structured zeolite with varied Si/Al ratios above 20can be produced with hierarchical porosity.3.3. SEM characterizationThe microscopic images measured by SEM are unraveled in Fig. 4.For Si/Al molar ratio of 20, conventional ZSM-5 are made up of uniformaggregates of 3–5 μm that are seen all over the sample. The surface ofconventional ZSM-5 appears corrugated, and the particles appear to be aggregates of tiny primary particulates ranging from 50 to 300 nm.Those primary particulates appear to be coffin-like in shape, which istypical for theMFI structure, but the sizes are atypically small. These ob-servations are in line with XRD results showing that peaks for Si/Al(molar ratio) = 20 are broadened substantially. The HPZ ZSM-5 witha Si/Al ratio of 20 possesses even smaller aggregates ranging from 200to 400 nm. The aggregates are also made up of even smaller primaryparticulates of 80 to 150 nm. The primary particulates in HPZ ZSM-5[Si/Al (molar ratio) = 20] are irregular in shape with respect to thosefor theHPZ counterpart. For Si/Al (molar ratio) samples, the charge den-sity of inorganic gel and SDAmatcheswell and a typical coffin-likemor-phology is observed for conventional ZSM-5. These particles are 2 to3 μm wide and 5 to 8 μm long, with clear edges and corners that indi-cates perfect crystallinity. The HPZ ZSM-5 with a Si/Al ratio of 180looks close to the previously reported ZSM-5 with a Si/Al ratio of 70[44], and aggregates of 200 to 400 nmare unanimously seen in the sam-ples. Close investigation of the surface shows that they are rough andmade of very fine primary particles of 20 to 40 nm. Between the primaryparticles, voids or interparticular spaces are found, which constitute themacro- or meso-pores that are measurable by N2 physisorptions.3.4. Infrared spectraFT-IR spectroscopy was also employed to identify the crystal phase,as depicted in Fig. 5. Contributions from ubiquitous H2O can be foundfrom the stretching bands of OH from 760 to 1660 cm−1. All samplesdisplay diagnostic ZSM-5 absorption bands at 1225, 1093 (strong),550 (media) and 450 cm−1(strong) [47]. The featured vibration bandat 550 cm−1had been assigned to the five-membered rings of T–O–T(T=Si or Al) inMFI-type zeolites, strongly indicating that all the frame-works aremade up of pentasil walls, in line with XRD designations [48,49]. The OH stretch vibrations normally appear in the range of 3000–3800 cm−1. The broad and shallow ones at ~3500 cm−1are generallyascribed to silanol nests that consist of a number of silanol groupsinteracting through hydrogen bonding [50]. The bands appear at3675 cm−1, corresponding to an overall contribution from externalfree silanols, internal silanols and bridging hydroxyls [50]. More silanolgroups as a result of the existence of silanol groups in different environ-ments are found for conventional ZSM-5 with Si/Al ratios of 20 and 40[50]. The evidences that there aremore defective sites of surface silanolsTable 1Surface area and pore structure analyses from N2 physisorptionSample SBET/m2•g−1Micropor. vol./ml•g−1①Mesopor. size/nm②Meso + macro-por./ml•g−1③ZSM-5-20 350 0.12 ––ZSM-5-HTS-20 567 0.11 7–30 0.71ZSM-5-40 386 0.12 ––ZSM-5-HTS-40 456 0.10 7–30 0.37ZSM-5-70 389 0.07 ––ZSM-5-HTS-70 468 0.11 11–30 0.36ZSM-5-180 350 0.10 ––ZSM-5-HTS-180 455 0.11 14–30 0.40① Inferred by t-plot method.② Deduced by employing a BJH method from the desorption branch of the isotherm;pore size distribution is also displayed in the inset of Fig. 2.③ Deduced by subtracting micropore volume from total pore volume. at lower Si/Al ratios are also confirmed by XRDmeasurements or
SEMob-servations. These defective sites are actually healed by the addition of HTSas lacking of such bands for HPZ ZSM-5 with similar compositions.3.5. NH3-TPD measurementsNH3-TPD technique is employed to identify the strength of acidity inZSM-5 and ZSM-5-HTS, and the profiles are displayed in Fig. 6.NH3,asatypical probe molecule to measure the strength of acidic sites, canpenetrate both micropores and larger mesopores, and therefore isused to give an overview of the acidic strength. NH3-TPD patterns canbe categorized into high temperature (N350 °C) and low temperature(b350 °C) ranges. It has been corroborated that weak acidity sitesfromb350 °C are fromphysisorbed ammonia on surface silanol groups,that are unimportant to catalysis. Peaks that desorb at N350 °C are fromNH4+ cations on the sites, and can be attributed to strong Brønsted acidicsites, which are catalytically active sites [51,52]. For conventional ZSM-5-20, a relatively large contribution from desorption of NH4+ from silanols is detected. The number ofweak silanols decreases progressive-ly with respect to a Si/Al ratio increase. The results show that there aremore silanols for low Si/Al ratios, which is also observed in IR measure-ments. The high temperature desorption of NH4+ from Brønsted acidicsites are also found in all conventional ZSM-5 samples, with desorptiontemperatures ranging from 400 to 500 °C. The small shift of desorptiontemperature for NH3 is tentatively ascribed to the existence of defectivesilanols, as also observed via XRD, IR or SEMcharacterizations. For ZSM-5-180, the high temperature desorption is hardly perceivable for thelow Al content and related low desorbed NH3 content. For ZSM-5-HTS,two distinct peaks can be discerned at 209 to 250 and 350 to 450 °Caswell. The lowtemperature desorption patterns are similar to the con-ventional ZSM-5. The profiles at high temperature ranges are almost su-perimposable, indicating very close acid strength for these ZSM-5-HTSsamples. In the synthetic protocol, we have employed TPAOH as struc-ture directing agents, which can cause Al zoning in the MFI crystal.This can complex the general trends in acidity with respect to Si/Alratios. In the case of HPZ MFI crystal, the presence of HTS can inhibitthe migration of Al in the crystal and heal the defective sites (in accor-dance with IR observations), and hence generates an obviously almostidentical acidity when the Si/Al ratio is above 10.3.6. Catalytic performanceThe catalytic test results of benzene methylation using ZSM-5with varied Si/Al ratios are presented in Fig. 7.Toluene(T)andxylene (X) are found to be the predominant products aftermethylation,other byproducts such as ethylbenzene, trimethylbenzene or methy-lethylbenzene are also detected in small amounts (sel.% = 7%–12%).According to a previously reported reaction mechanism and the factthat ZSM-5 is also a catalyst for methanol to olefin (MTO) conversion[5,23,24], a possible reaction pathway is illustrated in Schemes 1 and2. The reaction is initiated on Brønsted acid sites where protons onzeolites (Z⊝H⊕) attack methanol to generate surface Z⊝CH3⊕ and H2O.Z⊝CH3⊕ then attacks benzene to form protonated toluene. Deproton-ation produces toluene as the primary product and regenerate theBrønsted site to its original state (Fig. 8). Toluene can bemethylated to xy-lene or trimethylbenzenes via a similarmechanismafter sequentialmeth-ylations, as suggested by the reaction network in Fig. 9. At levatedtemperatures, it is proven that contribution from methanol to hydrocar-bons will occur to afford ethylene and some methylated benzenes [7].The presence of ethylene incurs benzene alkylation to ethylbenzene,which can further be methylated to methylethylbenzenes. Fig. 7 shows benzene conversion on ZSM-5 zeolites with different Si/Al ratios. For conventional ZSM-5, the conversion reduces from 41.0%(molar ratio of Si/Al is 20) to 38.3% (molar ratio of Si/Al is 40), before in-creasing to 43.1% (molar ratio of Si/Al is 70), and the conversion of 35.9% isreached at a Si/Al ratio of 180. A slow and continuous increase in tolueneselectivity from 60.1% to 69.2% is observed with the increase of the Si/Alratio for conventional ZSM-5, but the selectivity towards xylene decreasesfrom 28.3% (molar ratio of Si/Al is 20) to 23.2% at a Si/Al ratio of 180. Theabove results are close to previously reported values [5,23].ForHPZZSM-5, on the other hand, conversion of benzene increases from 41.4% (molarratio of Si/Al is 20) to 49.9% (molar ratio of Si/Al is 180) with a Si/Al ratioincrease. Simultaneously, selectivity to toluene exhibits a monotonicaldeclination from72.9% to 52.9%. At the same time, selectivity to secondarymethylation xylene increases from 38.5% to 43.8%.For both HPZ and conventional ZSM-5, the total selectivity of TX(toluene + xylene) is around 90%. The increase in conversion and TXselectivity for HPZ ZSM-5 suggests that trimethylbenzene selectivity isstill restricted as a result of shape selectivity imposed by themicroporesofMFI structure. The optimumTX yield of xylene is 34.9% for HPZwith aSi/Al ratio of 180, whereas the maximum TX yield is 25.6% for conven-tional ZSM-5