Insights into the nature of hydroxyl groups and Zn species over defective HZSM-5 zeolite supported zinc catalysts prepared by chemical liquid deposition ( CLD ) with Zn ( CH 2 CH 3 ) 2

A series of defective ZSM-5 zeolites (~300 nm, SiO2/Al2O3 ratio of 55, 100, 400 and 950) were intentionally prepared and systematically studied by XRD, SEM, Si MAS NMR, argon physisorption, NH3-TPD and FT-IR technologies. The nature, the amount and the accessibility of the acid sites of defective ZSM-5 zeolites are greatly different from reported ZSM-5 zeolites with perfect crystal structure. The co-existed strong Brønsted acid sites (Si(OH)Al) and weak Brønsted acid sites (hydroxyl nests) over defective ZSM-5 zeolites might lead to unique catalytic function. Zn(C2H5)2 was grafted onto defective ZSM-5 zeolites through chemical liquid deposition (CLD) method. Interestingly, FT-IR spectroscopy studies find that Zn(C2H5)2 was preferentially grafted on the hydroxyl nests with weak acidity rather than the Si(OH)Al groups with strong acidity over different defective ZSM-5 zeolites. Particularly, home-built operando dual beam FTIR-MS was applied to study the catalytic performance of Zn species locating at different sites of defective ZSM-5 zeolites under n-hexane transformation. Results show that Zn species grafted over hydroxyl nests obtain better dehydrogenation performance than Zn species over framework aluminum. This study provides guidance for the Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 3 December 2018 © 2018 by the author(s). Distributed under a Creative Commons CC BY license. rational design of highly efficient alkane dehydrogenative aromatization catalysts.


Introduction
The formation of a "hydroxyl-nest defect" over silica-alumina zeolites, consisting of 4 silanols, that substitute for [AlO 4 ] tetrahedron, was firstly proposed by Barrer and Makki [1].After that, Zecchina performed systematically theoretical calculation and found that the hydroxyl nests can be represented as one or more missing [SiO 4 ] units in the zeolitic framework.In order to the preserve the stoichiometry, the obtained microcavities are saturated with neighboring OH groups which are close enough to generate hydroxyl chains interacting through hydrogen bonding [2].Hydroxyl nests can form during the synthesis of zeolites, research has found that the amount of internal defects in zeolites depends reversely upon the concentration of Na and Al impurities [3], meanwhile it can be controlled by altering the silica source or changing fluoride as the mineralizing agent [4].Hydroxyl nests can also form in aluminosilicate or borosilicate zeolite materials by leaching or by steaming at high temperatures [5].
Zeolite material with hydroxyl nests is less hydrophobic than the zeolite with intact structure due to the presence of high density of internal OH groups [6].Heitmann et al [7] found these internal hydroxyl nests obtain very weak acidity which is suitable for special applications.For example, Beckman rearrangement reaction (cyclohexanone oxime to form ε-caprolactam in gas phase at 350 o C) could be catalyzed by the hydroxyl nests over silicalite-1 with high activity and selectivity [8].Besides providing weak acidity, the hydroxyl nests could also provide active sites and space for the grafting of heteroatoms such as Sn, Ti and B, etc [9].As we all known, the preparation of heteroatom substituted silica-alumina zeolites by directly synthesis is at low efficiency such as the preparation of Sn-Beta and TS-1 [10,11].The preparation procedure is usually complex and needs to be controlled strictly.
In comparison, grafting heteroatoms over hydroxyl nests by post-treatment could be operated at much milder conditions.
The special property of hydroxyl nests has been known for decades, however, the application of them has only been restricted in the Beckmann rearrangement.Jia et al [12] reported the hydroxyl nests are responsible for the rapid deactivation of ZSM-5 zeolite in methanol conversion.The hydroxyl nests accelerate the secondary reaction and coke formation side-reactions [13].
HZSM-5 zeolite with MFI structure (0.55*0.55 nm zigzag, 0.55*0.57nm straight channel) [14] has been widely applied in many heterogeneous catalytic processes [15].The well-developed porous structure with high surface area and strong acidity endows the extraordinary performance of HZSM-5 as successful solid acid catalyst.The acidity of HZSM-5 originates from the existence of framework aluminum species.Usually, the Si(OH)Al Brønsted acid sites located at the intersection of straight channel and zigzag channel of HZSM-5 zeolite obtain the strongest acidity while the Si(OH)Al Brønsted acid sites located at other sites obtain little weaker acidity.In general, the acid strength of HZSM-5 originates from the framework aluminum species (Si(OH)Al) is strong and the attenuation by post-treatment methods such as dealumination and ion-exchange by base metals is widely studied [16,17].Considering the weak acidity of hydroxyl nests, by regulating the ratio of hydroxyl nests to Si(OH)Al groups, we could prepare HZSM-5 zeolites with tunable acidity obtaining strong and weak acid sites simultaneously.These HZSM-5 zeolites could be named as defective zeolites and are expected to show novel catalytic properties.
In this study, a series of defective ZSM-5 zeolites with different SiO 2 /Al 2 O 3 ratio and hydroxyl nests content (~300 nm, SiO 2 /Al 2 O 3 ratio of 55, 100, 400 and 950) were intentionally prepared.Since the Zn modified HZSM-5 zeolites have been extensively studied and testified to be effective in the aromatization of short-chain hydrocarbons [18][19][20].Zn(C 2 H 5 ) 2 was grafted onto these defective ZSM-5 zeolites through CLD method.We find that Zn(C 2 H 5 ) 2 was preferentially grafted on the hydroxyl nests with weak acidity rather than on the Si(OH)Al groups with strong acidity over different defective ZSM-5 zeolites.The Zn species grafted over hydroxyl nests obtain better dehydrogenation performance than Zn species over framework aluminum species.were observed over all of these samples [21].The Z55 sample obtained the lowest relative crystallinity of 89.84%.In comparison, other as-prepared ZSM-5 zeolites (Z100, Z450 and Z900) obtain similar crystallinity with the commercial Z80 zeolite.For the as-prepared ZSM-5 zeolites, the surface area and pore volume increased with SiO 2 /Al 2 O 3 ratio (Table 1).Except for Z55, the other prepared zeolites obtain similar surface area and pore volume to that of Z80 reference.These results suggest that the as-prepared ZSM-5 zeolites with high SiO 2 /Al 2 O 3 ratio are well-crystallized zeolites with similar porous structure of commercial Z80 reference.
To our surprise, although the textual properties of the as-prepared ZSM-5 zeolites and Z80 reference are similar, their chemical properties are totally different.The acidity of these samples are characterized by OH-FTIR and NH 3 -TPD (Figure 3 and Figure S1). Figure 3a shows the characteristic IR spectra of the as-prepared ZSM-5 zeolites in the range of 3800-3200 cm -1 , which corresponds to the OH-stretch vibrations in ZSM-5 zeolites.Three adsorption bands could be identified from the spectra.The band at 3720 cm -1 belongs to the free internal silanol (SiOH) [22].The band at 3740 cm -1 can be associated with the isolated external silanol (SiOH), and the broad SiOH band centered at around 3500 cm -1 is generally ascribed to hydroxyl nests that consist of a number of silanol groups interacting through extended hydrogen bonding [22][23][24].The intensity of hydroxyl nests over different prepared ZSM-5 zeolites is great stronger than the other peaks.One thing needs to be paid attention is that the absorbance of framework aluminum species Si(OH)Al (3610 cm -1 ) [22,25] over these zeolites do not show up.These Si(OH)Al species might be overlapped by hydroxyl nests.Based on these OH-FTIR results, the as-prepared ZSM-5 zeolites containing substantial amount of hydroxyl nests were defined as defective ZSM-5 zeolites.
The NH 3 -TPD profiles of defective ZSM-5 zeolites with different the absorbance of framework aluminum species Si(OH)Al (3610 cm -1 ) is great stronger than that of the SiOH groups (Figure S1a).The strength and total amount of acid sites over Z80 is considerably larger than that of defective ZSM-5 zeolites.The resonance at -106 ppm is assigned to (AlO) 1 Si(OSi) 3 species [32] and the resonance at -102 ppm is assigned to (OH)Si(OSi) 3 species [33].The relative intensity of (AlO) 1 Si(OSi) 3 to (OH)Si(OSi) 3 over Z55 is great higher than the other samples which results from its high content of framework aluminum species (Si(OH)Al).For the other defective ZSM- In order to explore the potential of defective ZSM-5 zeolites as working catalyst and disclose the catalysis function of hydroxyl nests.Zn(C 2 H 5 ) 2 was grafted onto these defective ZSM-5 zeolites through CLD method.The interaction between Zn(C 2 H 5 ) 2 reactant and hydroxyl groups over zeolites is a stoichiometric reaction [34,35]  in Figure 5, for all the defective ZSM-5 zeolites, the Zn(C 2 H 5 ) 2 reactant preferentially reacted with the hydroxyl nests (the broad absorbance centered at 3500 cm -1 ).For Z55, the absorbance of hydroxyl nests of Zn/ZSM-5 gradually decreases with the increasing of Zn/Al ratio.Meanwhile, the absorbance of internal Si(OH) and external Si(OH) groups at 3720 cm -1 and 3740 cm -1 , respectively also decrease as the increasing Zn/Al ratio.
Interestingly, when the absorbance of hydroxyl nests vanishes, the absorbance of the Si(OH)Al at 3610 cm -1 shows up.Further increasing Zn/Al ratio, these  Figure 6 shows the 29 Si MAS NMR spectra of Zn modified Z950 zeolites.
The strongest resonance at -116 ppm corresponding to Si(OSi) 4 species over these samples is at the same level.While, the relative intensity of (AlO) 1 Si(OSi) 3 to (OH)Si(OSi) 3 over Z950 increased with the amount of grafted Zn.This suggest that Zn species was preferentially grafted on the hydroxyl nests over the defective ZSM-5 zeolite with high SiO 2 /Al 2 O 3 ratio.The results of FT-IR (Figure 5) and 29 Si MAS NMR (Figure 6) show that Zn(C 2 H 5 ) 2 preferentially interact with hydroxyl nests on defective ZSM-5 zeolites.In order to selectively graft zinc species on Si(OH)Al, eliminating the hydroxyl nests over defective ZSM-5 zeolite is necessary [34].Defective Z950 zeolite was treated by (NH 4 ) 2 SiF 6 solutions to prepare defect-free ZSM-5 zeolite named as Z950R [37], then Zn(C 2 H 5 ) 2 was grafted onto Z950R to obtain Zn/Z950R.As the OH-FTIR spectra shown in Figure 7, the hydroxyl nests (absorbance centered at around 3500 cm -1 ) disappeared while the Si(OH)Al species (absorbance at 3610 cm -1 ) shown up.These results suggest that the defects of Z950 were fully repaired after AHFS treatment and the main active sites of the as-prepared Z950R are Si(OH)Al and SiOH (internal and external).After Zn(C 2 H 5 ) 2 grafting, the absorbance at 3610 cm -1 corresponding to Si(OH)Al gradually decreased until fully disappeared when the Zn/Al ratio increased to 0.5.While the absorbance at 3720 and 3740 cm  The repairing of the framework of Z950 leads to the disappearance of hydroxyl nests and emergence of Si(OH)Al species (Figure 7) increasing the total amount of acid sites especially the acid sites with strong acid strength according to NH 3 -TPD results (Figure 8).For Z950, after Zn(C 2 H 5 ) 2 grafting, the amount of acid sites with medium strength gradually increased.For Z950R, after Zn(C 2 H 5 ) 2 grafting, the amount of acid sites with strong strength gradually decreased at the increase of acid sites with medium strength.The reason might lie in the ion exchange reaction between H + from Si(OH)Al and Zn 2+ which shifts the acid strength from strong to medium [38].were performed in pulse micro-reactor under atmospheric pressure.The product distributions of Zn/Z950 and Zn/Z950R were shown in Table S1.For Zn/Z950 (Figure 9), the selectivity of BTX apparently increased at the expense of the selectivity of C2-C6 olefin with Zn/Al ratio.While the selectivity of methane and ethane exhibited adverse trend.These results suggest that Zn 2+ located at hydroxyl nest could catalyze the dehydrogenative aromatization of n-hexane significantly.However, for Zn/Z950R, the selectivity of BTX slightly increased with Zn/Al ratio.These results show that Zn species located in hydroxyl nests obtain better dehydrogenation performance than Zn species over framework aluminum species.Operando DB-FTIR spectroscopy study was employed to know more about the catalytic functions of Zn 2+ locating at ion exchange position and nests in n-hexane transformation.To do this study, an operando spectroscopy system was built according to described method [39].The overall three-

Materials
Nano-sized defective NaZSM-5 zeolite with SiO In order to illustrate the role of defective sites over HZSM-5 in catalysis, the defective sites of ZSM-5 zeolite with SiO 2 /Al 2 O 3 ratio of 950 was repaired by (NH 4 ) 2 SiF 6 post treatment according to the publication [37].
The scanning electron microscopy (SEM) image to characterize the surface morphology of the as-prepared zeolite sample was taken on a Quanta 450 scanning electron microscope.
X-ray fluorescence (XRF) measurements were performed with a Bruker S8 TIGER spectrometer to determine the bulk silicon-to-aluminum ratio.
The molar ratio of Zn and Al was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) using an optima 2000DV windows.After the pretreatment, the cell was cooled down to room temperature for the sample measurements.The spectrum was recorded from 4000 to 400 cm -1 with an optical resolution of 4 cm -1 .The hydroxyl vibration spectra were obtained by subtracting the background spectrum (recorded with an empty IR cell in the absence of sample) from the measured sample spectra.
High resolution 29 Si magicangle spinning nuclear magnetic resonance (MAS-NMR) spectra were recorded using an Agilent DD2 500 spectrometer.compound and residual n-hexane, respectively.

Home-built operando dual beam FTIR-MS
In this case, a self-developed dual beam FT-IR spectrometer, a dual beam IR-cell reactor and an on-line mass spectrometer were used to construct the operando spectroscopy system.Catalyst samples were pressed into selfsupporting thin wafers (1 cm 2 ) and placed in the sample beam of the dual beam IR cell, and the reference beam was left vacant.The experiment procedures and the method to do spectrum subtraction were described elsewhere [22,39,42].In this study, samples were pretreated in the IR-cell reactor at 400 °C for 4 h under vacuum (10 -3 Pa).The temperature of n-hexane aromatization was carried out at 300 °C.N-hexane was carried into IR-cell reactor by nitrogen (10ml/min), GHSV = 220 h -1 .The spectra were recorded at a resolution of 4 cm -1 with 64 scans in the region of ṽ = 4000-400 cm -1 .
During the above FT-IR experiments the changes of the product composition have been monitored by a QMS 200 (Balzers) quadrupole massspectrometer.The changes in the signal intensity of the main fragments of benzene, ethylene, propene, methane, ethane, propane and those of the possible products were simultaneously followed.

Conclusions
A series of defective ZSM-5 with different SiO species grafted over framework aluminum species.This study provides guidance for the rational design of highly efficient alkane dehydrogenative aromatization catalysts.

Figure 11 .
Figure 11.Mass spectra of n-hexane aromatization on Zn 0.51 /Z950 and Zn 0.50 /Z950R catalysts.Reaction conditions: T = 300 °C, P = 101.33kPa, nhexane was carried into IR-cell reactor by N 2 (10ml/min), GHSV = 220 h -1 .During the above FT-IR experiment, the products were monitored by mass spectrometer, as shown in Fig 11, Zn 0.51 /Z950 exhibits higher olefins and aromatics composition than Zn 0.50 /Z950R, which indicates that Zn species located in hydroxyl nests obtained better dehydrogenation and aromatization properties than Zn species located in framework aluminum species.These results coincide quite well with the catalytic tests of pulse micro-reactor and operando dual beam FT-IR.

2 /
Al 2 O 3 molar ratio of 55, 100, 480 and 950 were manufactured by Dalian Ligong Qiwangda Chemical Technology (Dalian, China).The HZSM-5 was obtained by exchanging the NaZSM-5 twice at 80 o C with 1 M solution of NH 4 NO 3 with a liquid-to-solid weight ratio of 5 for 1 h.The sample was filtered and washed with deionized water each time after exchange, and finally dried at 110 o C for 12 h and then calcined at 540 o C in flowing dry air for 6 h.The prepared samples were named as Z55, Z100, Z480 and Z950.Commercial NH 4 ZSM-5 zeolite with SiO 2 /Al 2 O 3 molar ratio of 80 (CBV8014) was purchased from Zeolyst International to compare with the defective ZSM-5 zeolites.The ZSM-5 in hydrogen form was obtained by calcined at 540 o C in flowing dry air for 6 h, named as Z80.

3. 4 . Catalytic tests 3 . 4 . 1 .
Pulse micro-reactor N-hexane conversion tests were performed in a pulse micro-reactor under atmospheric pressure.The catalytic measurements were carried out at 500 and 600 o C respectively, the catalyst sample was pressurized to wafers and then crushed and sieved to 20-40 mesh before use.In a typical run, 200 mg of the zeolite catalyst was loaded, then 1 uL n-hexane was injected into the reactor with the contact reaction time about 0.15 s.The product composition was analyzed by an online gas chromatograph (TECHCOMP GC7900) equipped with a PLOT-Q column (30 m×4 mm) with a flame ionization detector.The n-hexane conversion (C n-hexane ) and product selectivity (S i ) were calculated using the following equations: where A i and A n-hexane are the corrected chromatographic areas of a specific Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 3 December 2018 Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 3 December 2018 doi:10.20944/preprints201812.0019.v1

2 /
Al 2 O 3 ratio were intentionally prepared and systematically titrated by Zn(C 2 H 5 ) 2 to study the difference between Zn species interacting with hydroxyl nests and Si(OH)Al by FT-IR and other technologies.Results show that Zn(C 2 H 5 ) 2 was preferentially grafted on the hydroxyl nests with weak acidity rather than Si(OH)Al groups with strong acidity over different defective ZSM-5 zeolites.Particularly, according to the operando dual beam FT-IR result, Zn species grafted on hydroxyl nests obtain better dehydrogenation performance than Zn Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 3 December 2018 Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 3 December 2018 doi:10.20944/preprints201812.0019.v1

Table 1 .
Textural properties of ZSM-5 zeolites with different SiO 2 /Al 2 O 3 ratios.: SBET, BET surface area was calculated by the Brunauer-Emmett-Teller (BET) method; Vmicro, micropore volume was determined by t-plot; Vmeso, mesopore volume was determined by Vtotal-Vmicro; the relative crystallinity was estimated by comparing the total XRD peak area of a zeolite sample in the range of 2 theta from 22 to 25 o with that of the parent HZSM-5 having the strongest diffraction intensity.The morphology of prepared ZSM-5 zeolites with different SiO 2 /Al 2 O 3 ratios was determined by SEM.As shown in Figure 1, all the crystals of these Note 5 zeolites with high SiO 2 /Al 2 O 3 ratios, although the OH-FTIR didn't find the Si(OH)Al groups, the Si(OH)Al indeed existed but greatly influenced by the hydroxyl nests.

3 December 2018 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 3 December 2018 doi:10.20944/preprints201812.0019.v1
as shown in Scheme 1.The Zn/Al ratio over Zn grafted different defective ZSM-5 zeolites was controlled at the same level.OH-FTIR was applied to reveal the relative activity of different hydroxyl groups with Zn(C 2 H 5 ) 2 over defective ZSM-5 zeolites.As the results shown Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: