Synthesis of HZSM-5 Rich in Paired Al and Its Catalytic Performance for Propane Aromatization

catalysts

Article Synthesis of HZSM-5 Rich in Paired Al and Its Catalytic Performance for Propane Aromatization

1,2, 1,2, 1, 1 1,2 1 Dezhi Shi †, Sen Wang † , Hao Wang *, Pengfei Wang , Li Zhang , Zhangfeng Qin , Jianguo Wang 1, Huaqing Zhu 1,* and Weibin Fan 1,*

1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, Shanxi, China; [email protected] (D.S.); [email protected] (S.W.); [email protected] (P.W.); [email protected] (L.Z.); [email protected] (Z.Q.); [email protected] (J.W.) 2 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (H.W.); [email protected] (H.Z.); [email protected] (W.F.) These authors equally contributed to this work. † Received: 26 May 2020; Accepted: 2 June 2020; Published: 3 June 2020

Abstract: A series of HZSM-5 catalysts with similar Si/AlF mole ratio, textual properties and morphology, but different contents of AlF pairs, were synthesized by controlling the Na/Al molar ratios in the precursor gel and used for propane aromatization. It is shown that the catalyst with a Na/Al molar ratio of 0.8 in the synthetic gel possesses the highest paired AlF concentration (64.4%) and shows higher propane conversion (38.2%) and aromatics selectivity (19.7 wt.%). Propane pulse experiments, micro reactor activity estimation, Operando diffuse reflectance ultraviolet-visible (DR UV-vis) spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) analysis of coke species deposited on the catalysts provide evidence that AlF pairs in the ZSM-5 framework promote oligomerization and cyclization reactions of olefins, and then produce more aromatics. Density Functional Theory (DFT) calculations demonstrate that the cyclization of olefins and hydride transfer reaction occurring on AlF pairs in HZSM-5 zeolite show a lower free energy barrier and a higher rate constant than those on single AlF, indicating that the structure of AlF pairs in the HZSM-5 zeolite has a stronger electrostatic stabilization effect on the transition states than that of single AlF.

Keywords: HZSM-5; Al pairs; propane aromatization; Operando DR UV-vis; DFT calculations

1. Introduction The aromatization of light alkanes (mainly propane and butane) has been extensively studied for almost three decades due to its importance in the economic and strategic fields [1,2]. For the propane aromatization process, Zn, Pt and Ga-promoted ZSM-5 catalysts are recognized as effective bifunctional catalysts [3–15]. Light alkane chain growth on bifunctional catalysts involves dehydrogenation, oligomerization, cyclization, and aromatization steps, which proceed in parallel with acid-catalyzed and thermal cracking reactions. In general, metal (Zn, Pt and Ga) species are responsible for dehydrogenation processes, while zeolitic acid sites serve catalytic functions for the oligomerization and cyclization steps [3,12,13]. ZSM-5 zeolite has been proved to be the most promising component of aromatization catalysts because of its high hydrothermal stability, unique shape-selective behavior, adjustable acidity and suitable crystal structure [1,3]. However, few studies have examined the influence of the framework aluminum (AlF) distribution of HZSM-5 zeolites on its propane aromatization catalytic performance. In the framework of HZSM-5, there are two types of Al species, i.e., paired Al and single Al. The term paired Al refers to the Al–O–(Si–O)1,2–Al sequences, whereas isolated single Al atoms represent Al–O–(Si–O) 3–Al sequences [16]. The content of AlF pairs and single AlF can be estimated ≥

Catalysts 2020, 10, 622; doi:10.3390/catal10060622 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 622 2 of 19 by using exchanged Co(II)-ZSM-5 samples. Hexaaquocomplexes of Co2+ ions can balance the negative charges generated by two close Al atoms, named as AlF pairs (located in one framework ring or different framework rings), whereas the single Al atom is defined as one Al atom sitting in a five- or 2+ six-membered ring that cannot be coordinated by Co ions [17,18]. AlF distribution can be regulated in the synthesis process of the ZSM-5 zeolites by changing the Si source (such as TEOS, colloidal silica and sodium silicate), Al source (AlCl3, Al(NO3)3, NaAlO2 and Al(OH)3), sodium content, and organic templating [17,19–21]. Biligetu et al. [19] reported that ZSM-5 zeolites with different framework Al achieved by adding various alcohols exhibit different methanol-to-olefins (MTO) performance, whereby a relatively large number of AlF pairs in the catalyst produces more ethene by enhancing the “aromatics-based cycle” [22]. Sazama and co-workers [20] found that AlF pairs in HZSM-5 framework were beneficial to 1-butene oligomerization in 1-butene conversion, whereas single AlF was favorable to 1-butene cracking and gave higher selectivity to lower olefins. Song et al. [23] reported that AlF pairs cooperatively catalyze alkane cracking at higher turnover rates than on single AlF and the apparent activation entropies become less negative at higher AlF pair concentrations although the apparent activation energies are similar on HZSM-5 catalysts with different concentrations of single AlF and AlF pairs on different alkane reactants (propane, n-butane, and n-pentane); thus, enhanced cracking rates at these AlF pairs are mainly due to more positive intrinsic activation entropies. Tabor et al. reported that turnover rates for the conversion of propene to C4-C9 olefins and to BTX aromatics are 5–8 times and 20 times higher, respectively, over AlF pairs than those over single AlF [24]. It is thus firmly believed that the distribution of AlF in HZSM-5 must be an important factor in catalytic activity and therefore, it is necessary to investigate the impact of AlF distribution in HZSM-5 on propane aromatization. In this work, a series of HZSM-5 catalysts with different contents of AlF pairs were synthesized by controlling the Na/Al molar ratios in the precursor gels and HZSM-5 catalysts with similar Si/AlF molar ratio, textual properties, and morphology were applied to propane aromatization reaction. It is found that the AlF pair content in HZSM-5 has a significant influence on propane conversion and selectivity to aromatics. Operando UV-vis spectroscopy and gas chromatography-mass spectrometry (GC-MS) and theoretical calculations revealed that AlF pairs in HZSM-5 framework promote oligomerization and cyclization reactions of olefins in propane aromatization. The structure of AlF pairs in the ZSM-5 zeolite shows stronger electrostatic stabilization effect on the transition states than that of single AlF. These results gave new insights into the impact of AlF distribution in HZSM-5 on propane aromatization, which should facilitate the design of high performance catalysts.

2. Results and Discussion

2.1. Physicochemical Properties of Catalysts The X-ray diffraction patterns of the catalysts are shown in Figure1. All of the catalysts exhibit the typical reflections of a MFI zeolite-type framework with similar crystallinity. The lattice parameters of the catalysts calculated trough Rietveld refinement of the XRD data are shown in Table1. All of the catalysts have similar cell parameters. The SEM images of the catalysts are shown in Figure2. All of the catalysts exhibit similar morphology and small uniform particles with an average size of ~100 nm. The subtle difference in these four samples is that pictures of samples HZ(0), HZ(0.4) and HZ(0.8) shows almost the same particle agglomerations while sample HZ(1.1) shows smaller particle agglomerations. Catalysts 2020, 10, x FOR PEER REVIEW 3 of 19

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Figure 1. XRD patterns of HZSM-5 catalysts with different Na/Al ratios in synthesis gels. (a) HZ(0), (b) HZ(0.4), (c) HZ(0.8), and (d) HZ(1.1).

Table 1. Crystallite and lattice parameters of catalysts.

Lattice Parameters Catalyst Crystal System a (Å) b (Å) c (Å) α β γ HZ(0) Orthorhombic 20.091 19.899 13.384 90° 90° 90° HZ(0.4) Orthorhombic 20.102 19.902 13.389 90° 90o 90° HZ(0.8) Orthorhombic 20.090 19.905 13.389 90° 90° 90° FigureFigure 1. 1.XRD XRD patterns patterns of of HZSM-5 HZSM-5 catalysts catalysts with with di differentﬀerent Na Na/Al/Al ratios ratios in in synthesis synthesis gels. gels. (a(a)) HZ(0), HZ(0), (b(b) HZ(0.4),) HZ(0.4),HZ(1.1) (c ()c HZ(0.8),) HZ(0.8), Orthorhombic and and (d ()d HZ(1.1).) HZ(1.1). 20.098 19.907 13.390 90° 90° 90°

Table 1. Crystallite and lattice parameters of catalysts.

Figure 2. SEM images of the HZSM-5 catalysts with diﬀerent Na/Al ratios in synthesis gels. (a) HZ(0), (b) HZ(0.4), (c) HZ(0.8), and (d) HZ(1.1).

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Table 1. Crystallite and lattice parameters of catalysts.

Lattice Parameters Catalyst Crystal System a (Å) b (Å) c (Å) α β γ

Catalysts 2020, 10HZ(0), x FOR PEER Orthorhombic REVIEW 20.091 19.899 13.384 90◦ 90◦ 90◦ 4 of 19 HZ(0.4) Orthorhombic 20.102 19.902 13.389 90◦ 90◦ 90◦ Figure 2. HZ(0.8)SEM imagesOrthorhombic of the HZSM-520.090 catalysts 19.905 with different 13.389 90Na/Al◦ ratios90◦ in 90synthesis◦ gels. (a) HZ(0), (b)HZ(1.1) HZ(0.4), (Orthorhombicc) HZ(0.8), and (d) 20.098HZ(1.1). 19.907 13.390 90◦ 90◦ 90◦

TheThe textual textual properties properties and and chemical chemical composition composition of of the the catalysts catalysts are are summarized summarized in in Table Table2. 2. TheThe catalysts catalysts have have similar similar surface surface areas, areas, pore pore volumes, volumes, pore pore size size distributions distributions and and Si /Si/AlAl ratios. ratios.

TableTable 2. Elemental2. Elemental composition, composition, textual textual properties properties and Al an distributiond Al distribution of HZSM of catalysts HZSM withcatalysts diﬀerent with Nadifferent/Al ratios Na/Al in synthesis ratios in gels. synthesis gels.

b c d e d AlF/Altotal b Stotal c Smicro Vtotale Vmicro Catalysts Si/Al a Si/AlF b Al /Al S S d V V d PSD f (nm) Catalysts Si/Al a Si/Al b (%)F total (m2gtotal−1) (mmicro2g−1) total(m3g−1) micro (m3g−PSD1) f (nm) F (%) (m2 g 1) (m2 g 1) (m3 g 1) (m3 g 1) HZ(0) 42 43 97.2 420 − 256 − 0.45− − 0.11 0.772~0.879 HZ(0.4)HZ(0) 37 4238 43 97.9 97.2 380 420 257 256 0.45 0.34 0.11 0.11 0.772~0.879 0.772~0.879 HZ(0.8)HZ(0.4) 38 3740 38 95.7 97.9 363 380 267 257 0.34 0.28 0.11 0.12 0.772~0.879 0.772~0.879 HZ(0.8) 38 40 95.7 363 267 0.28 0.12 0.772~0.879 HZ(1.1) 38 39 97.6 383 260 0.33 0.11 0.772~0.879 HZ(1.1) 38 39 97.6 383 260 0.33 0.11 0.772~0.879 a Measured by ICP-OES. b Determined by 27Al MAS NMR. c Total specific surface area determined by a Measured by ICP-OES. b Determined by 27Al MAS NMR. c Total speciﬁc surface area determined by BET equation. d e d MicroporeBET equation. surface Micropore area and micropore surface volumearea and determined micropore by volume t-plot method. determinede Total by pore t-plot volume method. determined Total bypore volume volume adsorbed determined at p/p0 = 0.99.by volumef Pore size adsorbed distribution at p/p (PSD)0 = was0.99. acquired f Pore bysize Density distribution Functional (PSD) Theory was (DFT)acquired method. by Density Functional Theory (DFT) method.

TheThe27 Al-MAS27Al-MAS NMR NMR spectra spectra of theof catalyststhe catalysts are shown are sh inown Figure in 3Figure. All samples 3. All exhibitsamples an exhibit intense an peakintense at about peak 55 at ppm, about ascribed 55 ppm, to tetrahedralascribed to framework tetrahedral Al framework and a weaker Al peakand ata approximatelyweaker peak at 0 ppmapproximately attributed to0 octahedralppm attributed extra-framework to octahedral Al. Itextra-framework is noticed that the Al. overwhelming It is noticed majority that ofthe 27 Aloverwhelming species is framework majority Al. of The Al Sispecies/AlF ratios is framework of the catalysts Al. The acquired Si/AlF fromratios ofAl-MAS the catalysts NMR analysis acquired closelyfrom match27Al-MAS their NMR Si/Al analysis ratios obtained closely bymatch ICP-OES, their indicatingSi/Al ratios that obtained almost by all ICP-OES, of Al atoms indicating are present that asalmost AlF in all the of ZSM-5 Al atoms framework. are present as AlF in the ZSM-5 framework.

Figure Figure3. 27Al-MAS 3. 27Al-MAS NMR spectra NMR spectraof HZ(0), of HZ(0.4), HZ(0), HZ(0.4), HZ(0.8) HZ(0.8) and HZ(1.1) and HZ(1.1) catalysts. catalysts.

Pyridine-adsorption infrared spectroscopy and NH3-TPD were employed to characterize the acidic properties of the catalysts, and the results are shown in Table 3. The catalysts possess similar numbers of Bronsted̈ and Lewis/weak and strong acid sites, which is in good agreement of the Si/AlF ratios. On the whole, the as-prepared catalysts exhibit similar morphology, textual properties, Si/AlF ratio and acidic property.

Table 3. Acidic properties of the catalysts based on pyridine adsorption IR and NH3-TPD analysis.

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Pyridine-adsorption infrared spectroscopy and NH3-TPD were employed to characterize the acidic properties of the catalysts, and the results are shown in Table3. The catalysts possess similar numbers of Brönsted and Lewis/weak and strong acid sites, which is in good agreement of the Si/AlF ratios. On the whole, the as-prepared catalysts exhibit similar morphology, textual properties, Si/AlF ratio and acidic property.

Table 3. Acidic properties of the catalysts based on pyridine adsorption IR and NH3-TPD analysis.

Acidity by Py-IR (mmol g 1) a Acidity by NH -TPD (mmol g 1) b Catalysts − 3 − Brønsted Lewis Total Strong Weak Total HZ(0) 0.20 0.04 0.24 0.32 0.28 0.62 HZ(0.4) 0.22 0.04 0.26 0.37 0.27 0.64 HZ(0.8) 0.22 0.05 0.27 0.36 0.27 0.63 HZ(1.1) 0.21 0.03 0.24 0.36 0.28 0.64 a The quantities of Brönsted and Lewis acid sites were calculated from the Py-IR spectra. b The quantities of weak and strong acid sites determined by NH3-TPD were measured by the amounts of ammonia desorbed at 120–250 and 250–500 ◦C, respectively.

2.2. Al distribution in the ZSM-5 Framework

The proportions of framework Al species (AlF pairs and single AlF) in the diﬀerent Co(II)-ZSM-5 samples are listed in Table4. It is found that the fraction of Al F pairs in as-prepared catalysts increases from 54.2% to 64.4% with the increase of the Na/Al molar ratio in the precursor gels from 0 to 0.8, and then decreases with further increase of the Na/Al ratio.

Table 4. Al distribution of Co-type ZSM-5 and HZSM-5.

Al Content (%) Al Pair Distribution (%) c Al Content by NMR (%) Samples F a a b d d Alsingle Alpairs (2Co + Na)/Al α β γ Al(54) Al(56) Co/ZSM-5(0) 45.8 54.2 0.99 14 64 22 31 29 Co/ZSM-5(0.4) 42.1 57.9 0.99 10 66 24 33 27 Co/ZSM-5(0.8) 35.6 64.4 0.96 10 65 25 33 27 Co/ZSM-5(1.1) 40.4 59.6 0.97 7 68 25 31 27 a The contents of single Al and Al pairs (2Co/Al) were calculated by the equations [Alsingle] = [Altotal]-2[Comax] and b [Alpairs] = 2[Comax], where Altotal and Comax were determined by ICP-OES. The (2Co + Na)/Al ratio was determined by ICP-OES. c The distribution of diﬀerent Al pairs types (α, β, and γ) was determined by the deconvolution of d Co(II) UV-vis-DR spectroscopy of Co-type ZSM-5 zeolites. The contents of Al(54) and Al(56) peaks were determined by deconvolution of the 27Al MAS NMR spectra of H-ZSM-5 catalysts.

Co(II) UV-vis-DRS was employed to investigate the distribution of AlF pairs in Co/ZSM-5 zeolites. The spectra were deconvoluted into seven bands of three Co(II) cations types coordinated with diﬀerent AlF pairs T-sites. The relative concentrations of α-type Co(II) ions in the straight channel, β-type Co(II) ions in the channel intersections and γ-type Co(II) ions in the sinusoidal channels are also given in Table4. Most of Al F pairs are distributed in the channel intersection T-sites (~66%), and the relative concentrations of these three types T-sites shows the similar tendence in all of the catalysts. 27Al MAS-NMR spectra were also employed to investigate the distribution of framework Al in HZSM-5 by deconvolution of the broad peak ranging from 45 to 65 ppm [17,21,25], as shown in Figure4. Among the ﬁve peaks, the peak at 54 ppm (Al (54)) and 56 ppm (Al(56)) are related to the framework Al sites in the channel intersections and in straight and sinusoidal channels, respectively. The deconvolution results are listed in Table4. Obviously, all the four samples exhibit a higher fraction of Al(54), suggesting that more AlF site are distributed in channel intersections. This is in agreement with the Co(II) UV-vis-DRS results. Catalysts 2020, 10, 622 6 of 19 Catalysts 2020, 10, x FOR PEER REVIEW 6 of 19

27 FigureFigure 4. 4.Deconvolution Deconvolution of of the the 27Al-MASAl-MAS NMRNMR spectraspectra ofof (a)) HZ(0), ( b) HZ(0.4), HZ(0.4), ( (cc)) HZ(0.8) HZ(0.8) and and (d) (d)HZ(1.1) HZ(1.1) catalysts. catalysts. 2.3. Catalytic Performance 2.3. Catalytic Performance 2.3.1. Catalytic Performance of HZSM-5 Zeolites in the Fixed-Bed Reactor 2.3.1. Catalytic Performance of HZSM-5 Zeolites in the Fixed-Bed Reactor The performance of the catalysts obtained in a fixed-bed reactor for propane aromatization is illustratedThe inperformance Figure5. The of propane the catalysts conversion obtained and in the a aromaticsfixed-bed selectivityreactor for exhibitpropane the aromatization same trends is illustrated in Figure 5. The propane conversion and the aromatics selectivity exhibit the same trends along with time on stream (TOS) for all catalysts. The catalyst HZ(0.8) with more AlF pairs shows muchalong higher with propanetime on conversionstream (TOS) and for aromatics all catalysts. selectivity The catalyst than the HZ(0.8) other catalysts. with more AlF pairs shows much higher propane conversion and aromatics selectivity than the other catalysts. The selectivity of products in propane aromatization as a function of AlF pairs content is illuminated in FigureThe6. Itselectivity can be seen of that products the aromatics in propane selectivity aromatization and hydrogen as a selectivity function areof Al increased,F pairs content but the is illuminated in= Figure= 6. It can be seen that the aromatics selectivity and hydrogen selectivity are selectivity to C3 -C4 olefins is decreased with the increasing AlF pairs content, while the selectivity to theincreased, cracking productsbut the selectivity (methane, ethaneto C3=-C and4= olefins ethene) is does decreased not diff erwith too much.the increasing The aromatics AlF pairs selectivity content, towhile HZ(0.8) the catalyst selectivity is 2.3 to times the highercracking than products that of HZ(0)(methane, catalyst. ethane What’s and ethene) more, the does coke not content differ oftoo themuch. used catalystsThe aromatics after 24 selectivity h TOS is alsoto HZ(0.8) shown catalyst in Figure is6 .2.3 It cantimes be seenhigher that than coke that content of HZ(0) deposited catalyst. What’s more, the coke content of the used catalysts after 24 h TOS is also shown in Figure 6. It can in the catalysts is increased with the increasing of AlF pairs content, which is plotted similar to the tendencebe seen of that aromatics coke content selectivity. deposited in the catalysts is increased with the increasing of AlF pairs content, which is plotted similar to the tendence of aromatics selectivity. Since the catalysts exhibit similar morphology, textual properties, Si/AlF ratio and Brønsted acid Since the catalysts exhibit similar morphology, textual properties, Si/AlF ratio and Brønsted acid sites numbers, the linear correlation between the selectivity of aromatics and olefins and AlF pairs contentssites numbers, reveals that the thelinear excellent correlation catalytic between performance the selectivity of HZ(0.8) of aromatics is probably and contributed olefins and to Al byF thepairs contents reveals that the excellent catalytic performance of HZ(0.8) is probably contributed to by the higher fraction of AlF pairs in the catalyst. higherThe fraction reaction of pathway AlF pairs ofin the propane catalyst. aromatization on HZSM-5 shown in Scheme1 is well established [3]. The primary products of propane aromatization are propene and hydrogen from the dehydrogenation process, in parallel with the formation of methane and ethylene through protonic cracking reactions. Propene undergoes cyclo-oligomerization and hydrogen transfer reactions to generate aromatics and alkanes at Brønsted acid sites on HZSM-5 catalysts. In the meantime, butene and pentene obtained from cracking of C6-C8 alkenes may oligomerize again to form C6-C8 alkenes.

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Therefore, the trend of selectivity to aromatics increase with the increase of AlF pair content accompanied with the decrease of oleﬁns production may be attributed to the presence of more AlF pairs in the catalyst, which enhances the oligomerization of alkenes and cyclization reactions to promote the

CatalystsformationCatalysts 2020 2020, 10 of,, x10aromatics. FOR, x FOR PEER PEER REVIEW The REVIEW following characterization and DFT calculation will demonstrate7 of7 this19of 19 hypothesis particularly.

Figure 5. Propane conversion (A) and aromatics selectivity (B) with the time on stream (TOS) over FigureFigure 5. Propane 5. Propane conversion conversion (A) ( Aand) and aromatics aromatics selectivity selectivity (B) (withB) with the thetime time on onstream stream (TOS) (TOS) over over HZSM-5 catalysts: HZ(0) (a), HZ(0.4) (b), HZ(0.8) (c) and HZ(1.1) (d). at 550 C with weight hourly HZSM-5HZSM-5 catalysts: catalysts: HZ(0) HZ(0) (a), (a),HZ(0.4) HZ(0.4) (b), (b), HZ(0.8) HZ(0.8) (c) and(c) and HZ(1.1) HZ(1.1) (d). (d). at 550at 550 °C◦ with°C with weight weight hourly hourly 1 space velocity (WHSV) of propane being 0.6− h1 − and N2/C3H8 ratio being 2.5 in feedstock. spacespace velocity velocity (WHSV) (WHSV) of propane of propane being being 0.6 0.6h hand−1 and N2/C N32H/C83 ratioH8 ratio being being 2.5 2.5in feedstock. in feedstock.

Figure 6. Product selectivity in propane aromatization at 20 h (TOS) and coke content in the used FigureFigure 6. Product6. Product selectivity selectivity in propanein propane aromatization aromatization at 20at h20 (TOS) h (TOS) and and coke coke content content in thein theused used catalysts as a function of AlF pairs content in the framework of HZSM-5 catalysts. catalystscatalysts as aas function a function of Al ofF AlpairsF pairs content content in the in theframework framework of HZSM-5 of HZSM-5 catalysts. catalysts.

TheThe reaction reaction pathway pathway of ofpropane propane aromatizat aromatizationion on onHZSM-5 HZSM-5 shown shown in inScheme Scheme 1 is1 wellis well establishedestablished [3]. [3]. The The primary primary products products of propaneof propane aromatization aromatization are are propene propene and and hydrogen hydrogen from from the the dehydrogenationdehydrogenation process, process, in parallelin parallel with with the the formation formation of methaneof methane and and ethylene ethylene through through protonic protonic crackingcracking reactions. reactions. Propene Propene undergoes undergoes cyclo-oligomerization cyclo-oligomerization and and hydrogen hydrogen transfer transfer reactions reactions to to generategenerate aromatics aromatics and and alkanes alkanes at Brønstedat Brønsted acid acid sites sites on onHZSM-5 HZSM-5 catalysts. catalysts. In theIn the meantime, meantime, butene butene andand pentene pentene obtained obtained from from cracking cracking of Cof6 -CC68-C alkenes8 alkenes may may oligomerize oligomerize again again to formto form C6 -CC68-C alkenes.8 alkenes. Therefore,Therefore, the the trend trend of ofselectivity selectivity to toaromatics aromatics increase increase with with the the increase increase of ofAl FAl pairF pair content content accompaniedaccompanied with with the the decrease decrease of olefinsof olefins production production may may be beattributed attributed to theto the presence presence of moreof more Al FAl F pairspairs in inthe the catalyst, catalyst, which which enhances enhances the the oligomerization oligomerization of ofalkenes alkenes and and cyclization cyclization reactions reactions to to promotepromote the the formation formation of ofaromatics. aromatics. The The following following characterization characterization and and DFT DFT calculation calculation will will demonstratedemonstrate this this hypothesis hypothesis particularly. particularly.

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Scheme 1. Reaction pathway of propane aromatization on HZSM-5 catalysts [3]. Scheme 1. Reaction pathway of propane aromatization on HZSM-5 catalysts [3]. 2.3.2. Micro Reactor Activity Estimation 2.3.2. Micro Reactor Activity Estimation In order to estimate the initial products of HZ(0) and HZ(0.8) catalysts at low propane conversion, In order to estimate the initial products of HZ(0) and HZ(0.8) catalysts at low propane the gaseous products at a time-on-stream of 2 min on a micro reactor at 500 ◦C were on-line analyzed. Propaneconversion, conversion the gaseous are limitedproducts to at 1.6% a time-on-stream and 1.8% for of HZ(0) 2 min andon a HZ(0.8)micro reactor catalysts, at 500 respectively. °C were Theon-line relative analyzed. concentrations Propane ofconversion gaseous productsare limited are to compared 1.6% and 1.8% in Table for 5HZ(0). and HZ(0.8) catalysts, respectively. The relative concentrations of gaseous products are compared in Table 5. Table 5. Relative concentration of gaseous products for propane aromatization obtained in micro Table 5. Relative concentration of gaseous products for propane aromatization obtained in micro reactor experiment. reactor experiment. Oleﬁn Concentrations a Catalysts Aromatics Concentration ( 104) a Olefin Concentrations a Catalysts Aromatics Concentration (×104) a × EtheneEthene Propene Propene Butene Butene HZ(0)HZ(0) 7.71 7.71 0.9240.924 0.159 0.159 0.015 0.015 HZ(0.8)HZ(0.8) 14.1 14.1 0.9180.918 0.089 0.089 0.014 0.014 a a Normalized by methane concentration in propane aromatization at 500 °C with the contact time of Normalized by methane concentration in propane aromatization at 500 ◦C with the contact time of propane being 1.5 s. propane being 1.5 s.

It canIt can be be seen seen that that the the relative relative concentration concentration of propene for for HZ(0.8) HZ(0.8) catalyst catalyst is ismuch much lower lower than than thatthat of of HZ(0) HZ(0) catalyst; catalyst; on on the the contrary, contrary, thethe relativerelative concentration of of aromatics aromatics for for HZ(0.8) HZ(0.8) catalyst catalyst is is muchmuch higher higher than than that that of of HZ(0) HZ(0) catalyst. catalyst. PropenePropene as the primary primary product product of of propane propane aromatization aromatization undergoesundergoes cyclo-oligomerization cyclo-oligomerization and and hydrogenhydrogen transfer reactions reactions to to generate generate aromatics aromatics and and alkanes alkanes at Brønstedat Brønsted acid acid sites sites (single (single Al AlF Fand andAl AlFF pairs)pairs) (Figure(Figure 77).). AlthoughAlthough both both single single Al AlF Fandand Al AlF pairsF pairs sitessites can can catalyze catalyze propene propene oligomerization, oligomerization, experimental experimental results results demonstrate demonstrate that catalyst that possessingcatalyst higherpossessing AlF pairs higher content AlF pairs produces content at theproduces expense at ofthe propene expense andof propene higher aromaticsand higher concentration aromatics inconcentration the products, in which the products, indicate which that the indicate presence that ofthe more presence AlF pairsof more in theAlF catalystspairs in the enhances catalysts the oligomerizationenhances the oligomerization of alkenes. of alkenes.

2.3.3.2.3.3. Pulse Pulse Experiment Experiment of of Propane Propane AromatizationAromatization ToTo investigate investigate thethe initialinitial products products retained retained in inthe the catalysts catalysts during during propane propane aromatization, aromatization, the thecatalytic catalytic performances performances of ofHZ(0) HZ(0) and and HZ(0.8) HZ(0.8) catalysts catalysts were were comparatively comparatively evaluated evaluated by bypulse pulse experiments at 500 °C. As illustrated in Figure 7, the products deposited on catalysts are mainly experiments at 500 ◦C. As illustrated in Figure7, the products deposited on catalysts are mainly benzene, toluene,benzene, xylenes, toluene, trimethylbenzenes xylenes, trimethylbenzenes (triMBs) and tetramethylbenzenes(triMBs) and tetramethylbenzenes (tetraMBs). Comparing (tetraMBs). with theComparing soluble coke with of the two soluble catalysts, coke the of HZ(0.8) two catalysts, catalyst the possesses HZ(0.8) morecatalyst aromatics, possesses especially more aromatics, benzene, especially benzene, toluene, xylenes. toluene, xylenes. The results of GC-MS and micro reactor activity estimation indicate more Al pairs in the catalysts enhance the oligomerization of alkenes (especially propene) and produce more aromatics, which is consistent with the results of catalytic performance tests.

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FigureFigure 7. 7.GC-MS GC-MS spectra spectra of theof retainedthe retained products products in HZ(0) in andHZ(0) HZ(0.8) and HZ(0.8) catalysts catalysts after pulse after of propanepulse of atpropane 500 ◦C, whichat 500 are°C, normalizedwhich are norma to thelized I.S.intensity. to the I.S. intensity.

2.3.4. Operando Diffuse Reflectance Ultraviolet-Visible (DR UV-vis) Spectra Experiment The results of GC-MS and micro reactor activity estimation indicate more Al pairs in the catalystsIt is well-known enhance the that oligomerization aromatization of of alkenes propane (e goesspecially through propene) dehydrogenation, and produce oligomerization, more aromatics, cyclization,which is consistent and aromatization with the results steps [of3,12 catalytic,13,24]. performance Taking into account tests. that propane dehydrogenation, as the first step of propane aromatization, can be catalyzed by metal species such as Zn, Pt and Ga etc.,2.3.4. but Operando the oligomerization Diffuse Reflectance and cyclization Ultraviolet-Visible steps of light (DR olefins UV-vis) are Spectra only catalyzed Experiment by zeolite acid sites, thusIt is Operando well-known diffuse that reflectance aromatization ultraviolet-visible of propane (DR UV-vis)goes through spectra experimentsdehydrogenation, were exploredoligomerization, to investigate cyclization, the formation and aromatization of hydrocarbon steps [3,12,13,24]. species (mainly Taking intermediate into account products) that propane for propenedehydrogenation, conversion. as the first step of propane aromatization, can be catalyzed by metal species such as Zn,Figure Pt and8 shows Ga etc., the but Operando the oligomerization DR UV-vis and spectra cyclization for conversion steps of light of propene olefins are at 250only catalyzed◦C over HZ(0)by zeolite and HZ(0.8)acid sites, catalysts, thus Operando respectively. diffuse The re bandsflectance located ultraviolet-visible at 220~300 nm (DR are UV-vis) attributed spectra to cyclopentadienes,experiments were cyclohexadienes, explored to investigate neutral (methylated) the formation benzenes, of hydrocarbon monoenyl carbocations species (mainly and alkyl-substitutedintermediate products) cyclopentenyl for propene carbocations conversion. [26 –30]. The bands centered at ca. 320~350 nm are ascribedFigure to dienyl 8 shows carbocations the Operando and lowDR UV-vis methylated spectr benzenea for conversion carbocations of propene (such as at xylenes 250 °C andover trimethylbenzenes)HZ(0) and HZ(0.8) [28 ,catalysts,31,32]. The respectively. bands at ca. 400~440The bands nm arelocated assigned at 220~300 to highly nm methylated are attributed benzene to carbocationscyclopentadienes, (such ascyclohexadienes, tetramethylbenzene, neutral pentamethylbenzene (methylated) benzenes, and hexamethylbenzene) monoenyl carbocations [28,30, 31and]. Analkyl-substituted increase in absorbance cyclopentenyl with increasing carbocations time-on-stream [26–30]. The is observed,bands centered indicating at ca. the 320~350 accumulation nm are ofascribed the intermediate to dienyl productscarbocations on the and zeolite low catalystsmethylated during benzene the conversioncarbocations of propene.(such as xylenes Compared and withtrimethylbenzenes) HZ(0) catalyst, more[28,31,32]. carbonaceous The bands materials at ca. are400~440 formed nm on are HZ(0.8) assigned catalyst, to highly especially methylated those locatedbenzene at 250~350carbocations nm assigned (such as to tetramethylbenze the intermediatesne, of aromaticspentamethylbenzene originated from and oligomerizationhexamethylbenzene) and cyclization[28,30,31]. processes.An increase The in result absorbance is in good with agreement increasing with time-on-stream the catalytic performance is observed, tests. indicating the accumulation of the intermediate products on the zeolite catalysts during the conversion of propene. Compared with HZ(0) catalyst, more carbonaceous materials are formed on HZ(0.8) catalyst, especially those located at 250~350 nm assigned to the intermediates of aromatics originated from oligomerization and cyclization processes. The result is in good agreement with the catalytic performance tests.

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Figure 8. Operando DR UV-vis spectroscopy for conversion of propene over HZ(0) and HZ(0.8) Figure 8. Operando DR UV-vis spectroscopy for conversion of propene over HZ(0) and HZ(0.8) Figurecatalysts 8. atOperando 250 °C. DR UV-vis spectroscopy for conversion of propene over HZ(0) and HZ(0.8) catalysts at 250 ◦C. catalysts at 250 °C. 2.4.2.4. CokeCoke AnalysisAnalysis 2.4. Coke Analysis DRDR UV-visUV-vis andand FTIRFTIR spectraspectra werewere employedemployed toto analyzeanalyze thethe cokecoke speciesspecies depositeddeposited inin thethe usedused catalysts.catalysts.DR UV-vis TheThe DR DR and UV-vis UV-vis FTIR spectra spectra of theofwere the used employed used HZ(0) HZ(0 and to) and HZ(0.8)analyze HZ(0.8) catalystthe cokecatalyst are species shown are shown deposited in Figure in Figure9 .in The the bands 9.used The catalysts.atbands ~330 andat The ~330 ~410 DR nmandUV-vis are~410 ascribedspectra nm ofare to the dienyl ascribed used carbocations HZ(0 to) anddienyl HZ(0.8)/low carbocations/low methylated catalyst are benzene shown methylated carbocations in Figure benzene 9. The and bandshighlycarbocations methylatedat ~330 and and highly benzene ~410 methylated carbocations,nm are benzeneascribed respectively carbocations, to dienyl [30 ]. respectivelycarbocations/low As for the band[30]. As atmethylated ~530for the nm, band itbenzene may at ~530 be carbocationscausednm, it bymay phenanthrene, andbe caused highly bymethylated anthracene phenanthrene, benzene carbocations, anthrace carbocations, depositedne carbocations, respectively pyrene and deposite /[30].or more Asd pyrenefor benzene the bandand/or rings-fused at ~530more nm,aromaticbenzene it may rings-fused molecules be caused [ 30aromatic by,31 ].phenanthrene, It molecules is noticed anthrace[30,31]. that the Itne coke is carbocations, noticed species that are depositethe similar coke ford species pyrene these are twoand/or similar catalysts. more for benzeneHowever,these two rings-fused one catalysts. cannot aromatic determineHowever, molecules preciseone cannot di [30,31].ﬀerences determ It betweenisine noticed precise the that didifferences ﬀtheerent coke reaction speciesbetween species are the similar with different TOS for thesefromreaction DRtwo species UV-vis catalysts. spectra.with However,TOS from DRone UV-vis cannot spectra. determ ine precise differences between the different reaction species with TOS from DR UV-vis spectra.

FigureFigure 9.9. DR UV-vis spectraspectra ofof thethe catalystscatalysts obtainedobtained inin didifferentfferent TOS.TOS. Figure 9. DR UV-vis spectra of the catalysts obtained in different TOS. InIn orderorder to to acquire acquire more more information information of reaction of reaction species species deposited deposited in the usedin the catalysts used catalysts at different at TOS,differentIn FTIR order TOS, spectroscopy to FTIR acquire spectroscopy wasmore carried information was out. carried Figure of reactionout. 10 depictsFigure species 10 the depicts FTIRdeposited spectra the FTIRin ofthe thespectra used used catalystsof HZ(0) the used and at differentHZ(0.8)HZ(0) and catalyst TOS, HZ(0.8) FTIR after reactionsspectroscopycatalyst after at di ffwasreactionserent carried TOS at (12 out.different and Figure 24 h), TOS denoted10 depicts(12 and as HZ(0)-12,the 24 FTIRh), denoted HZ(0)-24,spectra asof HZ(0.8)-12 HZ(0)-12,the used HZ(0)andHZ(0)-24, HZ(0.8)-24, and HZ(0.8)-12HZ(0.8) respectively. catalyst and HZ(0.8)-24, after reactions respectively. at different TOS (12 and 24 h), denoted as HZ(0)-12, HZ(0)-24,According HZ(0.8)-12 to previous and HZ(0.8)-24, researches respectively. [24,32–35], the bands at 1300–1500 cm−1 are not assigned, as a varietyAccording of bands to overlappedprevious researches in this region; [24,32–35], the band the bands at approximately at 1300–1500 1506,cm−1 are1520, not 1540, assigned, 1558 ascm a− 1 varietyare assigned of bands to overlappedmonoenyl carbeniumin this region; ions, the non- bandcondensed at approximately aromatics, 1506, alkylnaphthalenes 1520, 1540, 1558 and/or cm−1 are assigned to monoenyl carbenium ions, non-condensed aromatics, alkylnaphthalenes and/or

Catalysts 2020, 10, x FOR PEER REVIEW 11 of 19

polyphylenes; the band at ~1600 cm−1 is attributed to polyaromatics and/or condensed coke; another two bands at 1625 and 1635 cm−1 are due to double bonds or olefins. As shown in Figure 10A, the band at 1625 cm−1 are increased slightly for HZ(0.8)-12 sample compared to sample HZ(0)-12; the band at 1600 and 1625 cm−1 are rasied remarkable for HZ(0.8)-24 compared with HZ(0)-24, indicating that HZ(0.8) catalyst has more polyaromatics or double bonds coke species than HZ(0) catalyst at the same TOS. For HZ(0.8) catalysts at different TOS, polyaromatics and double bonds coke species are increased with increasing reaction time. As mentioned above, these two catalysts have similar textual properties and exhibit similar morphology, so coke contents were not affected by diffusion of products. This phenomenon is contributed to the higher fraction of AlF pairs in the catalyst, which enhances the oligomerization of alkenes and cyclization reactions to promote the Catalysts 2020, 10, 622 11 of 19 formation of aromatics.

Figure 10. FTIR spectra of the used HZ(0) and HZ(0.8) catalysts after propane aromatization for 12 and Figure 10. FTIR spectra of the used HZ(0) and HZ(0.8)1 catalysts after propane aromatization for 12 24 h at 550 ◦C with WHSV of propane being 0.6 h− and N2/C3H8 ratio being 2.5 in feedstock in the −1 and 24 h at 550 °C with WHSV1 of propane being 0.6 1h and N2/C3H8 ratio being 2.5 in feedstock in region of (A) 1700–1300 cm− and (B) 3100–2800 cm− for (a) HZ(0)-12, (b) HZ(0.8)-12, (c) HZ(0)-24, −1 −1 the region of (A) 1700–1300 cm and (B) 3100–2800 cm1 for (a) HZ(0)-12, (b) HZ(0.8)-12, (c) (d) HZ(0.8)-24; (C) Deconvoluted CH-region (3100–2800 cm− ) spectrum of HZ(0.8)-24; (D) Fraction of HZ(0)-24, (d) HZ(0.8)-24; (C) Deconvoluted CH-region (3100–2800 cm−1) spectrum of HZ(0.8)-24; (D) intensity (calculated using the area of the peaks in (B) of several characteristic vibrational bands of the Fraction of intensity (calculated using the area1 of the peaks in (B) of several characteristic vibrational catalysts in the FTIR region 2800–3100 cm− . bands of the catalysts in the FTIR region 2800–3100 cm−1. 1 According to previous researches [24,32–35], the bands at 1300–1500 cm− are not assigned, as a The bands between 2800 and 3100 cm−1 are assigned to aliphatic and aromatic C–H vibrations.1 variety of bands overlapped in this region; the band at approximately 1506, 1520, 1540, 1558 cm− areAs shown assigned in toFigure monoenyl 10B, the carbenium bands in ions,3000–3100 non-condensed cm−1 ascribed aromatics, to aromatic alkylnaphthalenes C–H vibration are and not/or 1 polyphylenes;observed in the the used band catalysts, at ~1600 probably cm− is attributed because most to polyaromatics of the deposited and/ orcoke condensed species are coke; fused-ring another 1 twoaromatics bands and/or at 1625 highly and 1635 branched cm− are cokes, due to which double exhibit bonds low or olefins. C–H intensity As shown [33,34]. in Figure 10A, the band 1 at 1625In cmorder− are to increasedquantify slightlythe coke for vibrations, HZ(0.8)-12 the sample spectrum compared of each to samplesample HZ(0)-12;is deconvoluted the band into at 1 −1 1600several and Lorentzian 1625 cm− arepeaks rasied [32,35]: remarkable 2850 forcm HZ(0.8)-24, –CH2 groups compared related with HZ(0)-24,to symmetric indicating paraffinic that HZ(0.8)hydrocarbons; catalyst 2875 has more cm−1, polyaromatics –CH3 groups or related double to bonds symmetric coke species paraffinic than HZ(0)hydrocarbons; catalyst at 2906~2937 the same TOS.cm−1, For–CH HZ(0.8) and –CH catalysts2 groups at di ffoferent asymmetric TOS, polyaromatics paraffinic andhydrocarbons; double bonds 2962 coke cm species−1, –CH are3 groups increased of withasymmetric increasing paraffinic reaction hydrocarbons; time. As mentioned 2980 cm above,−1, –CH these and two –CH catalysts2 groups have related similar to symmetric textual properties olefinic andhydrocarbons exhibit similar with morphology,high carbon sonumber. coke contents An exampl weree of not the aff ecteddeconvolution by diffusion is depicted of products. in Figure This phenomenon is contributed to the higher fraction of Al pairs in the catalyst, which enhances the F oligomerization of alkenes and cyclization reactions to promote the formation of aromatics. 1 The bands between 2800 and 3100 cm− are assigned to aliphatic and aromatic C–H vibrations. 1 As shown in Figure 10B, the bands in 3000–3100 cm− ascribed to aromatic C–H vibration are not observed in the used catalysts, probably because most of the deposited coke species are fused-ring aromatics and/or highly branched cokes, which exhibit low C–H intensity [33,34]. In order to quantify the coke vibrations, the spectrum of each sample is deconvoluted into several 1 Lorentzian peaks [32,35]: 2850 cm− , –CH2 groups related to symmetric paraffinic hydrocarbons; 1 1 2875 cm− , –CH3 groups related to symmetric paraffinic hydrocarbons; 2906~2937 cm− , –CH and 1 –CH2 groups of asymmetric paraffinic hydrocarbons; 2962 cm− , –CH3 groups of asymmetric paraffinic Catalysts 2020, 10, 622 12 of 19

Catalysts 2020, 10, x FOR PEER REVIEW 12 of 19 1 hydrocarbons; 2980 cm− , –CH and –CH2 groups related to symmetric olefinic hydrocarbons with 10Chigh where carbon the number. region An of example2800–3100 of cm the−1 deconvolution is plotted for isthe depicted HZ(0.8)-24 in Figure sample. 10C As where shown the in region Figure of 1 10D,2800–3100 the band cm− intensitiesis plotted forat 2962 the HZ(0.8)-24 cm−1 of used sample. HZ(0) As catalyst shown inare Figure much 10 higherD, the bandthan that intensities of used at 1 HZ(0.8)2962 cm −catalystof used at HZ(0) the catalystsame TOS, are muchwhile higher the band than thatintensity of used at HZ(0.8)2980 cm catalyst−1 show at thethe sameopposite, TOS, 1 whileindicating the band that intensityHZ(0.8) atcatalyst 2980 cm possesses− show theless opposite, paraffinic indicating hydrocarbons that HZ(0.8) species catalyst and more possesses olefinic less parahydrocarbonsffinic hydrocarbons species with species high andcarbon more number. olefinic Wi hydrocarbonsth the increase species of TOS, with the high band carbon intensity number. at 1 With2980 cm the−1 increase is increased of TOS, for the used band HZ(0.8) intensity catalysts, at 2980cm bu−t itis is increased decreased for for the the used used HZ(0.8) HZ(0) catalysts. catalysts, 1 Asbut mentioned it is decreased above, for this the usedband HZ(0)at 2980 catalysts. cm−1 is Asassigned mentioned to –CH above, and this –CH band2 groups at 2980 related cm− tois symmetricassigned to olefinic –CH and hydrocarbons –CH2 groups with related high to symmetriccarbon number, olefinic which hydrocarbons is oligomerized with high from carbon the oligomerizationnumber, which is of oligomerized lower olefins from in the the oligomerization propane aromatization. of lower olefins This in result the propane suggested aromatization. that the Thisoligomer result olefins suggested are decreased that the oligomer on HZ(0.8) olefins catalyst are decreased with the time on HZ(0.8) on stream. catalyst Combined with the with time the on resultstream. obtained Combined from with Figures the result 10A(b,d), obtained it fromis conv Figureinced 10 A(b,d),that the it ishigher convinced olefins that have the higherbeen further olefins convertedhave been into further aromatics converted on HZ(0.8) into aromatics catalyst on with HZ(0.8) the time catalyst on stream. with the time on stream. This FTIRFTIR analysisanalysis of of coke coke species species on theon usedthe used catalysts catalysts demonstrates demonstrates that higher that higher AlF pairs Al contentF pairs contentin the catalyst in the catalyst promotes promotes the oligomerization the oligomerization and cyclization and cyclization reactions reactions of olefins of olefins and and leads leads to the to productionthe production of more of more aromatics, aromatics, which iswhich in good is agreementin good agreement with the results with the of the results catalytic of the performance catalytic performancetests and Operando tests and DR Operando UV-vis spectra DR UV-vis experiments. spectra experiments.

2.5. DFT DFT Calculations DFT calculations were used to further evaluate the influence of the structure of single Al and DFT calculations were used to further evaluate the influence of the structure of single AlFF and Al pairs on the propane aromatization process. Here, 1-hexyl cation was applied as reactant to AlF pairs on the propane aromatization process. Here, 1-hexyl cation was applied as reactant to form benzene,form benzene, because because 1-hexyl 1-hexyl cation cationis a typical is a typicalprecursor precursor species (formed species (formedby propene by propene[36]) in propane [36]) in propanearomatization. aromatization. The aromatization The aromatization of 1-hexyl of 1-hexylcation cationundergoes undergoes cyclization, cyclization, hydride hydride transfer, transfer, and dehydrogenationand dehydrogenation to produce to produce aromatics. aromatics. As Asshown shown in inScheme Scheme 2,2 ,1-hexyl 1-hexyl cati cationon firstly firstly produces thethe cyclohexane through cyclization reaction (TS1). Th Then,en, the hydride transfer reaction (TS2) between cyclohexane andand propoxypropoxy leads leads to to the the formation formation of cyclohexylof cyclohexyl carbenium carbenium ion andion and propane. propane. After After that, that,the cyclohexyl the cyclohexyl carbenium carbenium ion is convertedion is converted to benzene to benzene by repeated by repeated deprotonations deprotonations and hydride and hydride transfer transferreactions reactions (TS3 and (TS3 TS4). and The TS4). calculated The calculated kinetic andkinetic thermodynamic and thermodynamic results areresults given are in given Table 6in. TableIt should 6. It beshould noticed be thenoticed deprotonation the deprotonation reaction reaction are not consideredare not considered because because the free the energy free energy barrier barrierof deprotonation of deprotonation reaction reaction is quite is low, quite in comparisonlow, in comparison with that with of cyclization that of cyclization and hydride and hydride transfer transferreactions reactions [37]. [37]. It was found that the ZSM-5 zeolite containing Al pairs shows lower free energy barrier and It was found that the ZSM-5 zeolite containing AlF pairs shows lower free energy barrier and higher rate constants of cyclization (171 kJ mol 1, 2.50 102 s 1) and hydride transfer (145–159 kJ mol 1, higher rate constants of cyclization (171 kJ −mol−1, 2.50× × 10− 2 s−1) and hydride transfer (145–159 −kJ 1.38 103–1.03 104 s 1) reactions than the ZSM-5 zeolite containing single Al (176 kJ mol 1 and mol−×1, 1.38 × 103×–1.03 ×− 104 s−1) reactions than the ZSM-5 zeolite containing single Al (176 kJ mol−−1 and 1.12 102 s 1 for cyclization, and 156–166 kJ mol 1 and 5.11 102–2.06 103 s 1 for hydride transfer). 1.12 ×× 102 s−1 for cyclization, and 156–166 kJ mol−−1 and 5.11 × 102–2.06 ×× 103 s−−1 for hydride transfer). Meanwhile, the low free energy barriers for cyclization and hydride transfer steps on ZSM-5 zeolite containing Al pairs, is mainly contributed to their lower enthalpy barriers. These results demonstrate containing AlF F pairs, is mainly contributed to their lower enthalpy barriers. These results that the structure of Al pairs in the framework of HZSM-5 zeolite shows stronger electrostatic demonstrate that the structureF of AlF pairs in the framework of HZSM-5 zeolite shows stronger stabilization effect on the transition states than that of single Al [38,39]. Therefore, it can better electrostatic stabilization effect on the transition states than that of Fsingle AlF [38,39]. Therefore, it can promotebetter promote the aromatization the aromatization process process and produce and produce more aromatic more aromatic species. species.

Scheme 2. ReactionReaction network network for for the the formation formation of of benz benzeneene in in the transition period for propane − aromatization overover H-ZSM-5H-ZSM-5 catalysts. catalysts. ZOH ZOH and and ZO ZO− represent represent the the protonated protonated and and deprotonated deprotonated acid acidsites sites of zeolite, of zeolite, respectively. respectively.

Table 6. Calculated free energy barriers (ΔGint≠), rate constants (k), enthalpy barriers (ΔHint≠), and entropy losses (−TΔSint≠), as well as reaction free energies (ΔGR) at 823 K for each reaction step in the formation of benzene in the transition period for propane aromatization over H-ZSM-5.

≠ ≠ ≠ Step ΔGint (kJ mol−1) k (s−1) ΔHint (kJ mol−1) −TΔSint (kJ mol−1) ΔGR (kJ mol−1) Single AlF TS1 176 1.12 × 102 157 19 −19

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, , Table 6. Calculated free energy barriers (∆Gint ), rate constants (k), enthalpy barriers (∆Hint ), and entropy losses ( T∆S ,), as well as reaction free energies (∆G ) at 823 K for each reaction step in − int R the formation of benzene in the transition period for propane aromatization over H-ZSM-5.

Step ∆G , (kJ mol 1) k (s 1) ∆H , (kJ mol 1) T∆S , (kJ mol 1) ∆G (kJ mol 1) int − − int − − int − R − Single AlF TS1 176 1.12 102 157 19 19 × − TS2 158 1.75 103 160 2 105 × − TS3 156 2.06 103 151 5 9 × TS4 166 5.11 102 158 8 1 × − AlF pairs TS1 171 2.50 102 151 20 28 × − TS2 145 1.03 104 154 9 108 × − TS3 149 5.34 103 143 6 27 × TS4 159 1.38 103 149 10 10 ×

3. Experimental Section

3.1. Catalyst Preparation

The ZSM-5 zeolites with diﬀerent contents of AlF pairs were synthesized by adjusting the Na/Al molar ratios (0, 0.4, 0.8 and 1.1) in the precursor gels. The Na-free gel contained tetraethyl orthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH, 25 wt.%) and aluminum nitrate (Al(NO3)3) in a composition proportion of 1 SiO2: 0.0125 Al2O3: 0.4 TPAOH: 66 H2O. The chemical composition + of Na -containing gels was 1 SiO2: 0.0125 Al2O3: 0.4 TPAOH: x NaCl: 36 H2O, where x was 0.01, 0.02 and 0.0275, corresponding to the samples with Na/Al ratios of 0.4, 0.8 and 1.1, respectively. The gel was stirred at room temperature for 12 h before being sealed into a Teﬂon-lined autoclave. After crystallation at 170 ◦C for 5 d, the material was centrifugated, washed with water, dried overnight at 100 ◦C, and calcined at 560 ◦C for 10 h in air to obtain the parent NaZSM-5 zeolite. H-form ZSM-5 was prepared by repeated ion exchanging NaZSM-5 zeolite with NH4NO3 aqueous solution (1 M, m(liquid)/m(solid) = 40) at 80 ◦C for 4 h, which was then calcined at 550 ◦C for 6 h in air. The samples were donated as HZ(m), where m is the Na/Al molar ratio in the precusor gel. The normal Si/Al molar ratio of all samples was 40. To investigate the Al distribution in the framework of ZSM-5 zeolites, Co-ZSM-5 samples were prepared by reverse ion exchanging of HZSM-5 with NaCl aqueous solution (1.0 M) at 80 ◦C for 6 h and subsequent ion exchanging of NaZSM-5 three times with a Co(NO3)2 aqueous solution (0.05 M) at 80 ◦C for 12 h under stirring condition. The Co-ZSM-5 sample was rinsed with distilled water three times, dried in air and calcined at 500 ◦C for 4 h in air [17].

3.2. Catalyst Characterization X-ray powder diffraction (XRD) patterns were collected on a MiniFlex II desktop X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (0.154 nm, 30 kV, 15 mA). The cell parameters of the catalysts were obtained by Rietveld refinement of XRD data using X’Pert HighScore Plus software (PANalytical, Holland). The scanning electron microscopy (SEM) images were taken on a field emission scanning electron microscope (FESEM, JSM 7001-F, JEOL, Japan). Nitrogen adsorption/desorption measurements were performed at 77 K on a TriStar II 3020 gas adsorption analyzer (Micromeritics, Norcross, GA, USA). Prior to the measurement, the zeolite sample was degassed at 300 ◦C for 8 h. The total specific surface area was calculated by Brunauer-Emmett-Teller (BET) method; the total pore volume was estimated from the volume of nitrogen adsorbed at a nitrogen relative pressure of 0.99. The pore size distribution was acquired by the Density Functional Theory (DFT) method. The micropore volume and external surface area were measured by t-plot method. The micropore surface area was obtained from the difference between the total pore surface area and the external surface area. Catalysts 2020, 10, 622 14 of 19

The chemical composition of the catalysts was determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Thermo iCAP6300, Thermo Fisher Scientific, Waltham, MA, USA). The ultraviolet-visible diffuse reflectance spectra (UV-vis-DRS) were measured on an Cary 5000 UV-vis-NIR spectrophotometer (Agilent, Wilmington, DE, USA) equipped with a poly (tetrafluoroethylene) integrating sphere. The UV-vis DRS of Co2+-exchanged samples were collected 1 at room temperature after dehydration at 773 K for 5 h under high vacuum condition (<10− Pa). The concentration of Alsingle and Alpairs was calculated using the following equations:

[Al ] = [Al ] 2[Comax] (1) single total −

[Alpairs] = 2[Comax] (2) where [Altotal] and [Comax] are the Al content and Co content in Co-type ZSM-5 zeolites, respectively, and both were determined by ICP-OES. FTIR spectra were measured on a Tensor 27 spectrometer (Bruker, Germany) in the range 1 1 400–4000 cm− with a resolution of 4 cm− by the conventional KBr method at room temperature and 2 10− Pa. Pyridine-adsorption infrared (Py-IR) spectra were measured on a Bruker Tensor 27 FTIR spectrometer. Before collecting the spectrum, the self-supported sample wafer was evacuated 2 at 350 ◦C and 10− Pa for 2 h, and cooled to room temperature subsequently. Then pyridine vapor was introduced into the sample cell for 1 h. The spectrum was collected after evacuation of the sample wafer at 150 ◦C for 1 h. The contents of Brønsted and Lewis acid sites were estimated using the following 1 equation by integrating the vibration bands at 1540 and 1450 cm− , respectively:

A S C = 1000 (3) ε × M ×

1 where C is the concentration of Brønsted and Lewis acid sites (µmol g− ), A is the area of the vibration 1 2 band at 1540 or 1450 cm− , S is the surface area of the sample wafer (1.33 cm ), ε is the molar extinction 1 coefficient (1.13 and 1.28 cm µmol− for Brønsted and Lewis acid sites, respectively), and M is the mass of sample (mg) [26]. Temperature-programmed desorption of NH3 (NH3-TPD) was performed on an AutoChem II 2920 chemisorption analyzer from Micromeritics (USA). A catalyst sample (0.1 g) was first pretreated at 550 ◦C for 2 h in Ar stream (30 mL/min) and then cooled to 120 ◦C. Then gaseous NH3 (5 vol % in argon, 30 mL/min) was introduced into the sample tube for 30 min to saturated adsorb of NH3 on the catalyst. After that, the physically adsorbed NH3 was removed by Ar (30 mL/min) at 120 ◦C for 2 h. To get the NH3-TPD profile, the catalyst was then heated from 120 to 500 ◦C at a ramp of 10 ◦C/min; the desorption signal was recorded by a thermal conductivity detector (TCD). Thermal gravimetric analysis (TGA) of the used catalysts were performed on a Rigaku Thermo plus Evo TG 8120 (Tokyo, Japan) instrument at a heating rate of 10 ◦C/min in an air flow (30 mL/min). The coke content deposited on the catalysts was determined by weight loss between 400 and 700 ◦C. 27Al MAS NMR spectra were measured on Bruker Avance III 600 MHz Wide Bore spectrometer (Bruker, Germany) operating at a magnetic field of 14.2 T. The spectra were acquired at a spinning rate of 13 kHz with a π/12 pulse width of 1.0 µs and a recycle delay of 1 s. Operando diffuse reflectance ultraviolet-visible (DR UV-vis) spectra were obtained using an Agilent Cary 5000 UV-vis-NIR spectrophotometer (Agilent, Wilmington, DE, USA). The self-supporting sample wafer (100 mg) was placed in a self-made quartz cell. The samples were pretreated at 250 ◦C under a flow of Ar (30 mL/min) at atmospheric pressure for 2 h. The spectra were obtained at 250 ◦C under a flow of propene (1 mL/min) at atmospheric pressure. Catalysts 2020, 10, 622 15 of 19

3.3. Catalysis tests

3.3.1. Propane Conversion in a Fixed-Bed Reactor The catalytic test for propane aromatization was carried out on a fixed-bed reactor (i.d. 10 mm) at atmospheric pressure. Fixed-bed reaction process was illustrated in Figure 11. Typically, 2.3 g of catalyst (20–40 mesh) was used per round. The reaction was carried out at 550 ◦C in a propane/nitrogen flow (42 mL/min) with a N2/C3H8 molar ratio of 2.5 viz. a weight hourly space velocity (WHSV) of Catalysts 2020, 10, x FOR PEER REVIEW1 15 of 19 propane being 0.6 h− . The gas and liquid products were separated with a cold trap. The gaseous products including H2, CH4, light olefins and paraffins were analyzed on-line on a gas chromatograph gas chromatograph (GC) (Agilent 7890A) equipped with a thermal conductivity detector (TCD, (GC) (Agilent 7890A) equipped with a thermal conductivity detector (TCD, MoleSieve 5A column, MoleSieve 5A column, 3 m × 1/16 inch × 1 μm) and two flame ionization detectors (FIDs, GC-OxyPlot 3 m 1/16 inch 1 µm) and two flame ionization detectors (FIDs, GC-OxyPlot column, 10 m 530 µm column,× 10 m × 530× μm × 10 μm; Al2O3 column, 50 m × 530 μm × 15 μm). The liquid+ products× 10 µm; Al2O3 column, 50 m 530 µm 15 µm). The liquid products including C5 non-aromatics including× C5+ non-aromatics and× aromatics× were analyzed by another Agilent 7890A gas and aromatics were analyzed by another Agilent 7890A gas chromatograph equipped with a FID and a chromatograph equipped with a FID and a capillary column (Agilent 19091S-001, 50 m × 200 μm × capillary column (Agilent 19091S-001, 50 m 200 µm 0.5 µm). The propane conversion and product 0.5 μm). The propane conversion and product selectivity× × were calculated as below: selectivity were calculated as below: ∑ 𝐶 𝐶 𝑋 = P × 100% (4) ∑C𝐶i Cpropane XPropane = P− 100% (4) Ci × 𝐶 𝑆 = Ci × 100% (5) S∑i =𝐶 P 𝐶 100% (5) C Cpropane × i − where Xpropane (%) is the conversion of propane, Si (wt.%) is the selectivity to the target product i; Ci where Xpropane (%) is the conversion of propane, Si (wt.%) is the selectivity to the target product i; Ci and Cpropane (wt.%) are the corrected mass concentration for species i and propane, respectively. and Cpropane (wt.%) are the corrected mass concentration for species i and propane, respectively.

Figure 11. Schematic diagram of ﬁxed-bed reaction process. Figure 11. Schematic diagram of fixed-bed reaction process. 3.3.2. Propane Conversion in a Micro Reactor 3.3.2. Propane Conversion in a Micro Reactor In order to estimate the initial products, conversion of propane was performed in a U-shaped quartzIn order tube to microestimate reactor the initial with anproducts, inner diameter conversion of 6of mm. propane About was 100 performed mg of catalyst in a U-shaped (20–40 mesh) quartz tube micro reactor with an inner diameter of 6 mm. About 100 mg of catalyst (20–40 mesh) was loaded in the reactor bed. The reaction was carried out at 500 °C in a propane/nitrogen flow (42 mL/min) as described above. The whole products at a time-on-stream of 2 min were analyzed on-line on a GC as mentioned above. Relative concentration of each product was obtained by dividing the calibrated chromatographic areas of the product by methane concentration since methane results mainly from protolytic cracking of propane on the zeolite protonic sites [3,40].

3.3.3. Pulse Reaction Test of Propane Conversion Pulse reaction test of propane conversion was also conducted in the U-shaped quartz tube reactor as mentioned above. The catalyst was pretreated in a N2 flow (20 mL/min) at 500 °C for 3 h.

Catalysts 2020, 10, 622 16 of 19

was loaded in the reactor bed. The reaction was carried out at 500 ◦C in a propane/nitrogen ﬂow (42 mL/min) as described above. The whole products at a time-on-stream of 2 min were analyzed on-line on a GC as mentioned above. Relative concentration of each product was obtained by dividing the calibrated chromatographic areas of the product by methane concentration since methane results mainly from protolytic cracking of propane on the zeolite protonic sites [3,40].

3.3.3.Catalysts Pulse 2020 Reaction, 10, x FOR TestPEERof REVIEW Propane Conversion 16 of 19 Pulse reaction test of propane conversion was also conducted in the U-shaped quartz tube reactor Then 2.0 mL propane was injected into the U-shaped quartz tube along with a N2 flow (20 mL/min). as mentioned above. The catalyst was pretreated in a N2 ﬂow (20 mL/min) at 500 ◦C for 3 h. Then 2.0 mL After 10 s, the carrier gas (N2) was switched out. The quartz tube was quenched in liquid nitrogen propane was injected into the U-shaped quartz tube along with a N2 ﬂow (20 mL/min). After 10 s, immediately after 70 s later of the injection. The retained products in the catalyst were dissolved in the carrier gas (N2) was switched out. The quartz tube was quenched in liquid nitrogen immediately 20 wt.% HF aqueous solution and subsequently extracted with CH2Cl2. The extracts, with after 70 s later of the injection. The retained products in the catalyst were dissolved in 20 wt.% HF hexachloroethane as an internal standard (I.S.), were analyzed on a Shimadzu GCMS-QP2010 gas aqueous solution and subsequently extracted with CH2Cl2. The extracts, with hexachloroethane as anchromatography-mass internal standard (I.S.), spectrometry were analyzed (GC-MS) on a Shimadzu system equipped GCMS-QP2010 with an gas Rtx-5MS chromatography-mass capillary column spectrometry(60 m × 0.25 (GC-MS)mm × 0.25 system μm). equipped with an Rtx-5MS capillary column (60 m 0.25 mm 0.25 µm). × × 3.4.3.4. DFT DFT Calculation Calculation TheThe catalytic catalytic kinetic kinetic and and thermodynamic thermodynamic parameters parameters (details (details see see Table Table6) of 6) each of each reaction reaction step step in aromatizationin aromatization process process were were calculated calculated on the on basis the of basis two 120of two T cluster 120 T models cluster containing models containing the single Althe andsingle Al pairs.Al and As Al shown pairs. in As Figure shown 12 ,in for Figure the model 12, for of singlethe model Al, one of Alsingle atom Al, substituted one Al atom one substituted Si atom at T12one site, Si atom whereas at T12 two site, Al atomswhereas substituted two Al atoms two Sisubs atomstituted at T12two and Si atoms T7 sites at toT12 form and the T7 sequencesites to form of Al-O-(Si-O)the sequence2-Al of in Al-O-(Si-O) one ring of2-Al zeolite in one framework. ring of zeolite The framework. peripheral silicon The peripheral atoms were silicon saturated atoms with were hydrogensaturated atoms; with hydrogen the distance atoms; between the distance hydrogen between and silicon hydrogen atoms and (Si–H silicon bond) atoms was (Si–H 1.47 Å, bond) and thewas direction1.47 Å, and of which the direction was the sameof which as that was of Si–Othe same bond. as Density that of functionalSi–O bond. theory Density (DFT) functional calculations theory of the(DFT) catalytic calculations kinetics of were the performedcatalytic kinetics with the were Gaussian performed 09.D01 with package the Gaussian [41]. Details 09.D01 can package be found [41]. in ourDetails former can work be found [26]. in our former work [26].

FigureFigure 12. 12.The The model model of of single single Al Al ( a()a and) and Al Al pairs pairs (b ()b of) of 120T 120T cluster cluster model model over over H-ZSM-5. H-ZSM-5. Atom Atom coloring:coloring: gray gray (Si), (Si), red red (O), (O), white white (H), (H), and and pink pink (Al). (Al). 4. Conclusions 4. Conclusions Series ZSM-5 catalysts containing diﬀerent AlF pair contents (54.2% to 64.4%) were synthesized Series ZSM-5 catalysts containing different AlF pair contents (54.2% to 64.4%) were synthesized by controlling the Na/Al molar ratios (0–1.1) in the precursor gels. The obtained ZSM-5 catalysts with by controlling the Na/Al molar ratios (0–1.1) in the precursor gels. The obtained ZSM-5 catalysts similar Si/AlF mole ratio, textual properties, morphology, but diﬀerent AlF pairs concentration were with similar Si/AlF mole ratio, textual properties, morphology, but different AlF pairs concentration applied to propane aromatization reactions. A linear correlation has been established between the were applied to propane aromatization reactions. A linear correlation has been established between the propane conversion/selectivity of aromatics and AlF pairs content. The catalyst HZ(0.8) (Na/Al molar ratio 0.8 in the gel) with the highest paired AlF concentration shows the highest propane conversion (38.2%) and aromatics selectivity (19.7 wt.%), which is about 1.5 times (for conversion) and almost 2.5 times (for aromatics selectivity) higher than that of HZ(0) catalyst. Propane pulse experiment, micro reactor activity estimation, Operando diffuse reflectance ultraviolet-visible (DR UV-vis) spectra and FTIR characterization of coke species on the used catalysts suggest that the present of more AlF pairs in the ZSM-5 framework promotes the oligomerization and cyclization reactions of olefins and leads to the production of more aromatics. DFT calculation results indicate that the HZSM-5 zeolite containing AlF pairs shows lower free energy barrier and higher rate constants for cyclization and hydride transfer reactions than the HZSM-5 zeolite containing single

Catalysts 2020, 10, 622 17 of 19

propane conversion/selectivity of aromatics and AlF pairs content. The catalyst HZ(0.8) (Na/Al molar ratio 0.8 in the gel) with the highest paired AlF concentration shows the highest propane conversion (38.2%) and aromatics selectivity (19.7 wt.%), which is about 1.5 times (for conversion) and almost 2.5 times (for aromatics selectivity) higher than that of HZ(0) catalyst. Propane pulse experiment, micro reactor activity estimation, Operando diffuse reflectance ultraviolet-visible (DR UV-vis) spectra and FTIR characterization of coke species on the used catalysts suggest that the present of more AlF pairs in the ZSM-5 framework promotes the oligomerization and cyclization reactions of olefins and leads to the production of more aromatics. DFT calculation results indicate that the HZSM-5 zeolite containing AlF pairs shows lower free energy barrier and higher rate constants for cyclization and hydride transfer reactions than the HZSM-5 zeolite containing single AlF; the structure of AlF pairs in the HZSM-5 zeolite exhibits stronger electrostatic stabilization effect on the transition states than that of single AlF. This study provided a theoretical guidance to design catalysts for propane aromatization.

Author Contributions: H.W., H.Z. and W.F. conceived and designed the experiments; D.S. carried out most of the experiments and characterizations; S.W. performed all the DFT calculations; D.S. and S.W. co-wrote the manuscript; P.W., L.Z., Z.Q. and J.W. discussed the results and gave many valuable suggestions for improving the work. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by the National Natural Science Foundation of China (21773281, 21991092, U1910203, 21802157, U1862101 and 21875275), Natural Science Foundation of Shanxi Province of China (201901D211581 and 201801D221092). Acknowledgments: The calculations are performed on the Computer Network Information Center of Chinese Academy of Sciences. Conflicts of Interest: The authors declare no conflict of interest.

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