Synthesis and Characterization of a New Aluminosilicate Molecular Sieve from Aluminosilica Perhydrate Hydrogel

materials

Article Synthesis and Characterization of a New Aluminosilicate Molecular Sieve from Aluminosilica Perhydrate Hydrogel

Haiqiang Ma 1, Kun Jiao 2, Xiangyu Xu 1 and Jiaqing Song 1,* 1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] (H.M.); [email protected] (X.X.) 2 College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China; [email protected] * Correspondence: [email protected]

Received: 6 October 2020; Accepted: 26 November 2020; Published: 30 November 2020

Abstract: A novel structure aluminosilicate molecular sieve, named BUCT-3, was prepared by dynamic hydrothermal synthesis, and the critical factor to obtain the new structure is using an active silicon and aluminum source, aluminosilica perhydrate hydrogel. Meanwhile, only high content of O-O bonds can ensure the pure phase of BUCT-3. Through the characterization of x-ray powder diﬀraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), and so on, some structure and morphology information of BUCT-3 molecular sieves as well as the special silicon and aluminum source was obtained. It’s worth noticing that the O-O bonds of reactants can be reserved in the products, and thus, help us to get a new structure with cell parameters a = 8.9645 Å, b = 15.2727 Å, c = 11.3907 Å, α = 90◦, β = 93.858◦, γ = 90◦. The crystal system is monoclinic. Though the thermostability of BUCT-3 is not satisfactory, its potential application derived from O-O bonds cannot be neglected.

Keywords: aluminosilica perhydrate hydrogel; molecular sieve; peroxides; silicon-aluminum source; synthesis method

1. Introduction Zeolites are microporous crystalline inorganic materials with well-defined pore systems. Since the 1940s, the study of zeolites with new structure has attracted a great deal of interest for their specific catalytic or adsorption properties [1,2]. Much effort has been devoted to developing novel molecular sieves [3,4]. Generally speaking, molecular sieves with new structures can be obtained in three ways: (i) Zeolites synthesized by using structure-tunable organic ammonium salt template: This method usually requires the use of organic amine/ammonium cations as structure directing agents (SDAs). Although designing new structures of templates is expected to be the most useful way to synthesize new structural molecular sieves, it’s also the most complicated method [5–7]. (ii) Molecular sieves synthesized by using heteroatom substitution [8]: heteroatom molecular sieves using other elements, such as Ti, Cr, Zr, and Ga [9–11], partially substitute silicon, aluminum, or phosphorus in the framework to form heteroatom-containing molecular sieves. These elements that enter the framework can be main group elements, or transition elements from 2+ to 5+. Due to the introduction of specific non-metal or metal atoms, many new molecular sieves with special structures have been successfully synthesized. (iii) Molecular sieves synthesized by topotactic transformation [12,13]: two steps are required to obtain three-dimensional structure molecular sieves from two-dimensional layered precursors: (a) Preparation of layered precursors by traditional hydrothermal synthesis;

Materials 2020, 13, 5469; doi:10.3390/ma13235469 www.mdpi.com/journal/materials Materials 2020, 13, 5469 2 of 14

(b) the two-dimensional precursor undergoes interlayer dehydration and condensation through high-temperature solid-phase reaction to form a three-dimensional molecular sieve, which is the process of topological transformation [5]. Many parameters can also influence the crystallinity, morphology, and even structure of molecular sieves, such as SDA/TO2, OH−/TO2, and H2O/TO2, as well as the synthesis temperature, crystallization time, and so on [14]. Another important feature of molecular sieves synthesis is that even if the other conditions of crystallization are the same, different framework structures may be obtained if different sources of reactants are used, especially the essential element of structure. It is known that the use of different silica sources may affect the type of zeolites [15–17], crystal size [17–25], morphology [20,21,25], and the rate of crystallization [15,16,19–24], since the fragile silicate intermediates releasing during the process of the silica dissolution plays an important role in the framework formation [26]. A typical example [27] is that scientists from Union Carbide Corporation converted an inert silicon-aluminum source into an active source, and synthesized a series of new molecular sieves ranging from A to Y, indicating that the activity of the source affects the structure of the molecular sieve. This work has inspired us to synthesize new molecular sieves by changing the activity of raw materials. Hydrogen peroxide is a clean and green chemical product. Its special electronic structure enables it to increase the activity of some reactions. Compared with the design of complex, expensive, and polluting organic templates to synthesize new molecular sieves, relatively inexpensive hydrogen peroxide is used to treat the reaction raw materials in order to change the activity of the source, so as to carry out the reaction process with low energy consumption. Considering cost-effectiveness and environmental cleanliness, it is advantageous to change the synthesis method of raw material activity to obtain new molecular sieves. In this article, a new kind of aluminosilicate molecular sieve with novel structure was obtained by using a usual template, tetrapropylammonium hydroxide (TPAOH) as the SDA, and unusual reactants, aluminosilica perhydrate hydrogel as the silicon and aluminum source. This is an unstable structure with peroxide bond that makes the structure elucidation very difficult. However, a combination of x-ray powder diffraction (XRD) and Fourier transform infrared spectra (FTIR) gives some partial structure of the novel material. We also explored in detail the effects of different silicon and aluminum sources, temperatures, and time on the products.

2. Materials and Methods

2.1. Chemicals

The chemicals used in our study were as following: tetraethoxysilane (TEOS, 28 wt% SiO2, Guang Dong Fine Chemicals Co., Ltd., Guangdong, China), tetrapropylammonium hydroxide (TPAOH 25 wt% in water, Shanghai Aladdin Bio-Chem Technology Co., LTD, Shanghai, China), Ammonia solution (NH3 25–28 wt%, Beijing Chemical works Co., Ltd., Beijing, China), hydrogen peroxide (H2O2 30 wt%, Beijing Chemical works Co., Ltd., Beijing, China) and deionized water, sodium aluminate (Al2O3 50–56 wt%, Na2O 40–45 wt%, Fisher, UK), and sodium silicate (Na2SiO3, Guang Dong Fine Chemicals Co., Ltd., Guangdong, China).

2.2. Synthesis Silica gel: A certain amount of ammonium fluoride was dissolved in deionized water according to the molar ratio of 0.004:4 to obtain solution A. Then, solution A was added dropwise to Solution B consisting of ethanol, water, and TEOS in a molar ratio of 8:4:1. After stirring until uniform, a transparent gel is obtained, which is dried in an oven at 50 ◦C for 1 d, and then roughly ground after taking it out. Finally, it is calcined in a muffle furnace at 800 ◦C for 2 h. Aluminosilica perhydrate hydrogel-1 (SiAl-1): Aluminosilica perhydrate hydrogel-1 was prepared by dissolving a fixed amount of NaAlO2 and Na2SiO3 (Si/Al = 25) in deionized water, and then H2O2 Materials 2020, 13, 5469 3 of 14 were added dropwise to the above solution and stirred for 1 h at room temperature. After drying at 50 ◦C for 24 h, the product was named as SiAl-1. The initial feed ratio of the gel was: Al2O3:SiO2:H2O2:H2O = 1:50:194:2162. Aluminosilica perhydrate hydrogel-2 (SiAl-2): TEOS was dissolved in the mixture of ethanol and deionized water, followed by a small amount of ammonia solution to promote the hydrolysis process. Subsequently, NaAlO2 solution was added to the silica gel with fixed Si/Al ratio 25. After stirring for a few minutes, H2O2 was then added dropwise to the mixture with continuous stirring at room temperature for 0.5 h. After drying at 50 ◦C for 24 h, the product was named as SiAl-2. The initial feed ratio of the gel was: Al2O3:SiO2:H2O2:H2O = 1:50:365:1720. Preparation of aluminosilicate molecular sieves: All samples were prepared by dynamic hydrothermal synthesis. The mixture of TPAOH, silica source, aluminum source, and deionized water was directly transferred into a teflon autoclave (25 mL capacity) with continuous stirring for at least 6 h. Finally, the autoclave was transferred to a homogeneous reactor at 70–120 ◦C for 2–4 days. Then, solid products were centrifuged, followed by repeatedly washing with deionized water and drying at 60 ◦C overnight. The initial feed ratio of the mixture was Al2O3 SiO2:TPAOH:H2O = 1:50:25:2500.

2.3. Characterizations The X-ray diffraction (XRD) measurement was performed to obtain the phase information of the molecular sieve samples. The XRD data were collected by UItima III diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation, the scanning range is from 5◦ to 50◦, scanning 3 times at a rate of 1 10◦ min− . Scanning electron microscopy (SEM) images were performed using ZEISS SUPER55 scanning electron microscope (ZEISS, Jena, Germany). Its acceleration voltage is 3 kV, and the magnification is 1 103–1 106. × × Infrared spectra (IR) were collected on a Vector 22 FTIR spectrometer (Bruker, Leipzig, Germany) 1 to obtain some structural information of the new molecular sieve (400–5000 cm− ). In-situ variable temperature X-ray diffraction data were collected on a modified Bruker D8 Advance diffractometer equipped with MRI high temperature attachment, a graphite monochromator 1 and LynxEye detector (Bruker, Leipzig, Germany), using a Cu Ka radiation source ( 4 1.5418 Å) in Bragg-Brentano geometry, with a step width of 0.02◦ in the 2θ range from 5◦ to 50◦, and the keeping 1 time was 2 s per step. The heating rate was 10 ◦C min− , and the sample was equilibrated for 300 s prior to XRD data collection at a certain temperature. Element analysis of products were characterized by elemental analysis (EA) using a Vario El elemental analyzer (elementar Analysensyteme GmbH, hanau, Germany). X-ray photoelectron spectroscopy (XPS) measurements conducted by AXIS Ultra DLD (Kratos, Manchester, UK) with Al Kα radiation (149.94 W).

3. Results and Discussion As mentioned in the introduction, different silica or aluminum sources strongly influence the nucleation and crystallization of zeolites. Our initial experimental idea was to study the effect on ZSM-5 by using different silicon and aluminum sources. Yet, considerable work has examined the influence of common silicon sources, like tetraethylorthosilicate (TEOS), fumed silica, and colloidal silica, sodium metasilicate [28]. For this reason, we carefully selected a special silicon source and aluminum source, aluminosilica perhydrate hydrogel, to investigate the influence on the structure of ZSM-5. At the same time, by adjusting the synthesis conditions, we hope to synthesize a molecular sieve with a new structure.

3.1. Characterization of Raw Materials Prior studies [29,30] have reported methods for preparing silica-alumina gel but there are few reports on the preparation of aluminosilica perhydrate hydrogel. In fact, the preparation method of the Materials 2020, 13, 5469 4 of 14

Materials 2020, 13, x FOR PEER REVIEW 4 of 14 aluminosilicaMaterials 2020, 13 perhydrate, x FOR PEER hydrogelREVIEW is similar with the silica-alumina gel, except that part of the water4 of in14 the silica-alumina gel was replaced by hydrogen peroxide solution when the aluminosilica perhydrate water in the silica-alumina gel was replaced by hydrogen peroxide solution when the aluminosilica hydrogelwater in the was silica-alumina prepared. Peralumina gel was replaced or persilica by ishydrogen a complexation peroxide of aluminasolution andwhen hydrogen-peroxide, the aluminosilica perhydrate hydrogel was prepared. Peralumina or persilica is a complexation of alumina and whichperhydrate contain hydrogel peroxide was or prepared. hydroperoxide Peralumina groups. or In persilica our experiment, is a complexation two kinds of aluminosilicaalumina and hydrogen-peroxide, which contain peroxide or hydroperoxide groups. In our experiment, two kinds perhydratehydrogen-peroxide, hydrogel which were contain obtained, peroxide namely or SiAl-1hydroperoxide and SiAl-2, groups. which In diourﬀered experiment, in the silicon two kinds and of aluminosilica perhydrate hydrogel were obtained, namely SiAl-1 and SiAl-2, which differed in the aluminumof aluminosilica source perhydrate used in the hydrogel process. were Figure obtained,1 shows thenamely XRD SiAl-1 of SiAl-1 and and SiAl-2, SiAl-2; which the bulgingdiffered peaksin the silicon and aluminum source used in the process. Figure 1 shows the XRD of SiAl-1 and SiAl-2; the showsilicon a and clear aluminum amorphous source phase. used Figure in 2the shows process. SEM Figure images 1 ofshows the silicon the XRD and of aluminum SiAl-1 and source SiAl-2; SiAl-1 the bulging peaks show a clear amorphous phase. Figure 2 shows SEM images of the silicon and andbulging SiAl-2. peaks The show images a ofclear both amorphous materials demonstrate phase. Figure spherically 2 shows shaped SEM images nanoparticles of the ofsilicon 20–50 and nm. aluminum source SiAl-1 and SiAl-2. The images of both materials demonstrate spherically shaped Obviously,aluminum thesource sample SiAl-1 of SiAl-2and SiAl-2. is more The uniform images and of dispersal.both materials Owing demonstrate to the hydrolysis spherically rate of shaped TEOS nanoparticles of 20–50 nm. Obviously, the sample of SiAl-2 is more uniform and dispersal. Owing to innanoparticles water is slow, of 20–50 the reaction nm. Obviously, process is the easy sample to control, of SiAl-2 and is the more prepared uniform colloidal and dispersal. particles Owing have the to the hydrolysis rate of TEOS in water is slow, the reaction process is easy to control, and the prepared samethe hydrolysis size without rate agglomeration.of TEOS in water is slow, the reaction process is easy to control, and the prepared colloidal particles have the same size without agglomeration. colloidal particles have the same size without agglomeration.

Figure 1. X-ray powder diffraction (XRD) patterns of sample (a) SiAl-1 and (b) SiAl-2. Figure 1. X-ray powder diffraction diﬀraction (XRD) patterns of sample (a) SiAl-1 and ((b)) SiAl-2.SiAl-2.

Figure 2. Scanning electron microscopy (SEM) images of sample (a) SiAl-1 and (b) SiAl-2. Figure 2. Scanning electron microscopy (SEM) images of sample (a) SiAl-1 and (b) SiAl-2. Figure 2. Scanning electron microscopy (SEM) images of sample (a) SiAl-1 and (b) SiAl-2. The FTIR spectroscopy is an effective method for studying hydrogen peroxide complexes. Due The FTIR spectroscopy is an effective method for studying hydrogen peroxide complexes. Due to theThe unique FTIR electronic spectroscopy structure is an eandffective geometric method co fornfiguration studying hydrogenof the hydrogen peroxide peroxide complexes. molecule, Due toit to the unique electronic structure and geometric configuration of the hydrogen peroxide molecule, it thecan uniqueform a strong electronic hydrogen structure bond and with geometric the hydrox configurationyl group on of the the surface hydrogen of the peroxide silica gel molecule, [31]. To can form a strong hydrogen bond with the hydroxyl group on the surface of the silica gel [31]. To itconfirm can form the aO-O strong bonds hydrogen were actually bond with formed the hydroxylin the prepared group aluminosilica on the surface perhydrate of the silica hydrogel, gel [31]. confirm the O-O bonds were actually formed in the prepared aluminosilica perhydrate hydrogel, ToSiAl-1 confirm and SiAl-2 the O-O sample bonds was were studied actually by the formed FTIR in technique. the prepared Figure aluminosilica 3 shows theperhydrate FTIR spectra hydrogel, of SiAl- SiAl-1 and SiAl-2 sample was studied by the FTIR technique. Figure 3 shows the FTIR spectra of SiAl- SiAl-11 and SiAl-2. and SiAl-2 The hydrogen sample was bond studied formed by by the the FTIR O-H technique. groups (Si-OH) Figure with3 shows H2O theand FTIRH2O2 spectramolecules of 1 and SiAl-2. The hydrogen bond formed by the O-H groups (Si-OH) with H2O and H2O2 molecules SiAl-1was found and to SiAl-2. have a The strong hydrogen absorption bond peak formed near 3500 by the cm O-H−1. However, groups the (Si-OH) bending with vibration H2O and of HH-O-2O2 was found to have a strong absorption peak near 3500 cm−1. However, the 1bending vibration of H-O- moleculesO (H2O2) connected was found to the to havesilica aframework strong absorption was found peak to be near weakly 3500 absorbed cm− . However, at 1300 cm the−1. Previous bending O (H2O2) connected to the silica framework was found to be weakly absorbed at 1300 cm−1. Previous vibrationstudies [32,33] of H-O-O show (Hthat2O the2) connected sharp peak to at the 876 silica cm−1 framework should be assigned was found to (O-O) to be weaklystretching absorbed vibration at studies [32,33] show that the sharp peak at 876 cm−1 should be assigned to (O-O) stretching vibration of silica perhydrate, relatively speaking, the alumina perhydrate spectrum shows a broad peak here. of silica perhydrate, relatively speaking, the alumina perhydrate spectrum shows a broad peak here. Moreover, compared with SiAl-1, SiAl-2 has a relatively sharper peak, which means the concentration Moreover, compared with SiAl-1, SiAl-2 has a relatively sharper peak, which means the concentration

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1 1 1300 cm− . Previous studies [32,33] show that the sharp peak at 876 cm− should be assigned to (O-O) stretching vibration of silica perhydrate, relatively speaking, the alumina perhydrate spectrum shows aMaterials broad peak2020, 13 here., x FOR Moreover, PEER REVIEW compared with SiAl-1, SiAl-2 has a relatively sharper peak, which means5 of 14 the concentration of O-O bond of SiAl-2 is higher than that of SiAl-1, since the sample/KBr ratio was of O-O bond of SiAl-2 is higher than that of SiAl-1, since the sample/KBr ratio was fixed in all the ﬁxed in all the FTIR analysis. FTIR analysis.

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of O-O bond of SiAl-2 is higher than that of SiAl-1, since the sample/KBr ratio was fixed in all the FTIR analysis.

FigureFigure 3. Fourier 3. Fourier transform transform infrared infrared spectra spectra (FTIR)(FTIR) spectra spectra of of sample sample (a) SiAl-1(a) SiAl-1 and (andb) SiAl-2. (b) SiAl-2.

In order to study the species and relative content of oxygen in aluminosilica perhydrate hydrogel, In orderFigure to 3. study Fourier the transform species andinfrared relative spectra content (FTIR) of spectra oxygen of insample aluminosilica (a) SiAl-1 and perhydrate (b) SiAl-2. hydrogel, high-resolutionhigh-resolution XPS XPS spectra spectra of of O O were were used used for for peak peak fitting. fitting. Figure Figure4 shows 4 shows XPS ofXPS sample of sample (a) SiAl-1 (a) SiAl-1 and (b)and SiAl-2. (b) SiAl-2.In As order can As to becanstudy seen be the seen from species from the and the figure, relative figure, the content the spectra sp ectraof oxygen of of SiAl-1 SiAl-1 in aluminosilica and and SIAl-2 SIAl-2 perhydrate can can be be fitted fitted hydrogel, into into three three high-resolution XPS spectra of O were used for peak fitting. Figure 4 shows XPS of sample (a) SiAl-1 componentscomponents with peak peak positions positions of of 530.1, 530.1, 531.2, 531.2, and and 532.2 532.2 eV. According eV. According to previous to previous research research reports and (b) SiAl-2. As can be seen from the figure, the spectra of SiAl-1 and SIAl-2 can be fitted into three [34,35], the peak position of O22− 1s was2 found at 530 eV. The energy peak positions at 531 and 532 eV reportscomponents [34,35], the with peak peak position positions of of O 2530.1,− 1s 531.2, was foundand 532.2 at 530eV. eV.According The energy to previous peak positionsresearch reports at 531 and should correspond to the surface OH- and bulk T-O-T (T = Si, Al). At the same time, we have 532 eV[34,35], should the correspond peak position to of the O22 surface− 1s was OH-found and at 530 bulk eV. T-O-T The energy (T = Si,peak Al). positions At the at same 531 and time, 532 we eVhave summarizedsummarizedshould correspond thethe OO 1s1s content contentto the surface ofof di differentff erentOH- and species species bulk in inT- SiAl-1 O-TSiAl-1 (T and and= Si, SiAl-2 SiAl-2Al). At according according the same to time,to the the we quantitative quantitative have 2 2− calculationcalculationsummarized methodmethod the ofOof XPS,1sXPS, content as as shown shown of different in in Table Table species1. 1. It inIt can SiAl-1can be be clearlyand clearly SiAl-2 seen seen according from from the tothe tablethe table quantitative that that the the O 2O−2 contentcontentcalculation inin SiAl-2 SiAl-2 method isis much much of XPS, higher higher as thanshown than SiAl-2. SiAl-2. in Table ThisThis 1. It maymay can bebebe dueclearlydue toto seen thethepreparation frompreparation the table methodmethod that the of ofO SiAl-2,22SiAl-2,− thatthat is, is,content the the continuous continuous in SiAl-2 is dropwise muchdropwise higher addition addition than SiAl-2. of of hydrogen hydr Thisogen may peroxide peroxidebe due to during theduring preparation the the TEOS TEOS method hydrolysis hydrolysis of SiAl-2, process process cancan firmly firmlythat is, lock lockthe continuous the the peroxide peroxide dropwise bond. bond. addition ThisThis isiscompletely completelyof hydrogen consistentperoxideconsistent during withwith the the TEOS resultsresults hydrolysis ofof infraredinfrared process analysis.analysis. Consequently,Consequently,can firmly welock we successfully thesuccessfully peroxide incorporatedbond. incorporated This is completely the the O-O O-O bond consistentbond into into thewith the aluminosilica thealuminosilica results of infrared gel. gel. analysis. Consequently, we successfully incorporated the O-O bond into the aluminosilica gel.

FigureFigure 4. X-ray 4. X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) of of sample sample (a ()a SiAl-1) SiAl-1 and and (b) (SiAl-2.b) SiAl-2. Figure 4. X-ray photoelectron spectroscopy (XPS) of sample (a) SiAl-1 and (b) SiAl-2. Table 1. O 1s content of different species in SiAl-1 and SiAl-2 from XPS. Table 1. O 1s content of different species in SiAl-1 and SiAl-2 from XPS. Sample O22− (%) OH− (%) Bulk O (%)

Sample O22− (%) OH− (%) Bulk O (%)

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Table 1. O 1s content of diﬀerent species in SiAl-1 and SiAl-2 from XPS.

2 Sample O2 − (%) OH− (%) Bulk O (%) SiAl-1 2.1 21.6 76.1 SiAl-2 23.4 33.2 42.3

3.2. Optimized Synthesis and Characterization ZSM-5 is a zeolite with MFI framework structure, usually prepared by hydrothermal synthesis, the reaction conditions were as follows: Al2O3:SiO2:TPAOH:H2O = 1:10–60:10–50:200–3000 at 100–175 ◦C for 1–8 days [36,37]. In our experiments, tetrapropylammonium hydroxide was chosen as the SDA. Two highly active aluminosilica perhydrate hydrogel were selected as the source. By contrast, silica gel and NaAlO2 were selected as the common source of silica-aluminum. Since the silicon and aluminum source have peroxide groups, which have relatively high reactivity, the reaction should better be performed at a low temperature, like 90 ◦C. In order to intuitively express the synthesis conditions and results, we list the source, temperature, time, and phase in Table2.

Table 2. The reaction conditions and the phases of Sample 1–9 (Al2O3:SiO2:TPAOH:H2O = 1:50:25:2500, 30 r/min).

Silicon and Sample Number Temperature ( C) Time (d) Phase Aluminum Source ◦

Sample 1 SiO2 + NaAlO2 70 4 amorphous Sample 2 SiO2 + NaAlO2 90 2 amorphous Sample 3 SiO2 + NaAlO2 90 4 amorphous Sample 4 SiAl-1 70 4 NaP + BUCT-3 Sample 5 SiAl-1 90 2 NaP + BUCT-3 Sample 6 SiAl-1 90 4 NaP + BUCT-3 Sample 7 SiAl-2 70 4 BUCT-3 (L) Sample 8 SiAl-2 90 2 BUCT-3 Sample 9 SiAl-2 90 4 BUCT-3 L: low crystallinity.

The SEM images of Sample 1–9 are shown in Figure5. Notably, the morphologies of products obtained by using diﬀerent silicon and aluminum sources are diﬀerent. Thereinto, Samples 1–3 are nanosized particles, while Samples 4–6 are a mixture with two morphologies, including the spherical morphology and plate-like morphology. As for Samples 7–9, a plate-like morphology with lamellar structure is formed. It can be seen from Figure6 that except for the samples prepared with SiO 2 and NaAlO2 as raw materials (Sample 1, 2, and 3) are amorphous, other samples have obvious characteristic peaks of BUCT-3 molecular sieve at 2θ = 9.68, 13.4, and 24.78◦, whereas, the samples (Sample 4, 5, and 6) with SiAl-1 as source have characteristic peaks of NaP zeolite at 2θ = 12.51, 17.74, 21.71, 28.2, and 33.5◦. However, no impurity peak (Sample 7, 8, and 9) was present in the XRD patterns when SiAl-2 was chosen as the starting source. Meanwhile, the crystallinity from Sample 7 to Sample 9 increased with increase of reacting time and temperature, which can be clearly seen from the height of peak at 2θ = 9.68◦. Materials 2020, 13, 5469 7 of 14

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Figure 5. SEM images of Sample 1–9 with different reaction conditions. Figure 5. SEM images of Sample 1–9 with diﬀerent reaction conditions.

ItAs can shown be seen in Table from2 ,Figure there are 6 that three except kinds for of phasesthe samples in the prepared products. with In addition SiO2 and to theNaAlO amorphous2 as raw materialsform, there (Sample are a known 1, 2, and phase 3) are NaP amorphous, zeolite, and other a new samples phase of have unknown obvious structure, characteristic named peaks BUCT-3. of BUCT-3It’s worth molecular noticing sieve that this at 2 newθ = 9.68, kind 13.4, of molecular and 24.78°, sieve whereas, can always the samples be synthesized (Sample when4, 5, and the 6) silicon with SiAl-1 as source have characteristic peaks of NaP zeolite at 2θ = 12.51, 17.74, 21.71, 28.2, and 33.5°. and aluminum sources were treated with H2O2 under all conditions. On the contrary, when common However, no impurity peak (Sample 7, 8, and 9) was present in the XRD patterns when SiAl-2 was silicon and aluminum sources, SiO2 and NaAlO2, were used in these conditions, the products were chosenalways as amorphous. the starting This source. implies Meanwhile, that at this the temperature, crystallinity from due to Sample the low 7 to activity Sample of 9 ordinary increased source, with increasethe nucleation of reacting and growthtime and of temp crystalserature, are slower,which can and be the clearly final seen appearance from the is height amorphous. of peak Overall, at 2θ = 9.68these °. results are in excellent agreement with the SEM images observed. Undeniably, SiAl-1 and SiAl-2 As shown in Table 2, there are three kinds of phases in the products. In addition to the with O-O bond formed by the addition of H2O2 into the silica-alumina gel have higher reactivity. amorphousThat is, the O-Oform, bond there plays are aa crucialknown role phase in the NaP formation zeolite, ofand the a newnew molecularphase of sieves.unknown structure, namedIt isBUCT-3. well known It’s worth that apart noticing from that crystallinity this new andkind morphology, of molecular the sieve crystal can phasealways is be also synthesized affected by whentemperature. the silicon In and order aluminum to further sources increase were the treated crystallinity with H of2O BUCT-3,2 under all the conditions. temperature On wasthe contrary, raised to when common silicon and aluminum sources, SiO2 and NaAlO2, were used in these conditions, the 120 ◦C; the phase composition is shown in Table3. Figure7 shows the SEM images of Sample 10 and products were always amorphous. This implies that at this temperature, due to the low activity of 11 prepared with different silica-alumina sources at 120 ◦C. It can be seen that both samples are sphere ordinarymorphology source, packed the withnucleation small particles,and growth and of the cr wholeystals crystalare slower, size is and about the 1 µfinalm. Figureappearance8 shows is amorphous. Overall, these results are in excellent agreement with the SEM images observed. the XRD patterns of both samples and they have typical MFI structure. It is clearly seen that further Undeniably, SiAl-1 and SiAl-2 with O-O bond formed by the addition of H2O2 into the silica-alumina increase in temperature led to the appearance of ZSM-5 zeolites, which is contrary to our expectation. gel have higher reactivity. That is, the O-O bond plays a crucial role in the formation of the new This is because that the O-O bond of SiAl-2 may break at the temperature 120 ◦C, and therefore, no O-O molecular sieves. bond was involved in the reaction, which eventually led to the presence of ZSM-5. This result proves once again that the silica-alumina with O-O bond plays an important role in the formation of the structure of BUCT-3.

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Figure 6. XRD patterns of Sample 1–9.

It is well known that apart from crystallinity and morphology, the crystal phase is also affected by temperature. In order to further increase the crystallinity of BUCT-3, the temperature was raised to 120 °C; the phase composition is shown in Table 3. Figure 7 shows the SEM images of Sample 10 and 11 prepared with different silica-alumina sources at 120 °C. It can be seen that both samples are sphere morphology packed with small particles, and the whole crystal size is about 1 μm. Figure 8 shows the XRD patterns of both samples and they have typical MFI structure. It is clearly seen that further increase in temperature led to the appearance of ZSM-5 zeolites, which is contrary to our expectation. This is because that the O-O bond of SiAl-2 may break at the temperature 120 °C, and therefore, no O-O bond was involved in the reaction, which eventually led to the presence of ZSM-5. This result proves once again that the silica-alumina with O-O bond plays an important role in the

formation of the structure of BUCT-3. FigureFigure 6. 6.XRD XRD patterns patterns of of Sample Sample 1–9.1–9. Table 3. The reaction conditions and phases of Samples 10 and 11. It is well known that apart from crystallinity and morphology, the crystal phase is also affected by temperature. InTable order 3. toThe further reaction increase conditions the crystallinity and phases of of Samples BUCT-3, 10 the and temperature 11. was raised Silicon and Sampleto 120 °C; Number the phase composition is shown in TableTemperature 3. Figure (°C) 7 shows theTime(d) SEM images ofPhase Sample 10 AluminumSilicon Source and andSample 11 prepared Number with different silica-aluminaTemperature sources at 120 ( C) °C. It can Time(d) be seen that both Phase samples are Aluminum Source ◦ sphereSample morphology 10 packedSiO2 + with NaAlO small2 particles, and 120the whole crystal size2 is about 1 μZSM-5m. Figure 8 shows Samplethe XRD 10 patterns of SiO both2 + NaAlOsamples2 and they have120 typical MFI structure. 2 It is clearly ZSM-5 seen that Sample 11 SiAl-2 120 2 ZSM-5 furtherSample increase 11 in temperatureSiAl-2 led to the appearance120 of ZSM-5 zeolites, which2 is contraryZSM-5 to our expectation. This is because that the O-O bond of SiAl-2 may break at the temperature 120 °C, and therefore, no O-O bond was involved in the reaction, which eventually led to the presence of ZSM-5. This result proves once again that the silica-alumina with O-O bond plays an important role in the formation of the structure of BUCT-3.

Table 3. The reaction conditions and phases of Samples 10 and 11.

Silicon and Sample Number Temperature (°C) Time(d) Phase Aluminum Source

Sample 10 SiO2 + NaAlO2 120 2 ZSM-5

Sample 11 SiAl-2 120 2 ZSM-5 Figure 7. SEM images of Samples 10 and 11. Figure 7. SEM images of Samples 10 and 11.

Figure 7. SEM images of Samples 10 and 11.

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FigureFigure 8. 8. TheThe XRD XRD patterns patterns of of Samples Samples 10 10 and and 11. 11.

3.3.3.3. Structural Structural Analysis Analysis of of BUCT-3 BUCT-3 Molecular Molecular Sieves Sieves BasedBased on on the the discussion discussion above, above, when when SiAl-2 SiAl-2 was was chosen chosen as as the the silicon silicon and and aluminum aluminum source source and and thethe reacting reacting condition condition was was set set as as 90 90 °C◦C for for 4 4 days, days, th thee product product has has the the highest highest crystallinity. crystallinity. Therefore, Therefore, SampleSample 9 9 was was used used to to solve solve the the structure structure of of BUCT-3 molecular sieves. TheThe indexing indexing of of Powder Powder X-ray X-ray Diffraction Diffraction data data of of BUCT-3 BUCT-3 was was carried carried out out by by Fullprof Fullprof program program andand the the cell parameters areare listedlisted inin Table Table4. 4. Since Since the the result result cannot cannot match match any any known known structure structure in the in theInternational International Zeolite Zeolite Association Association (IZA) (IZA) database database [38 ],[38], that that is, nois, structures’no structures’ cell cell parameters parameters have have the thesame same or multipleor multiple relationship relationship with with BUCT-3’s, BUCT-3’s, we we can can conclude conclude that that this this is a is new a new structure. structure. Since Since the thecrystal crystal size size of the of materialthe material cannot cannot meet themeet conditions the conditions of single of crystalsingle dicrystaffraction,l diffraction, the crystal the structure crystal structurecannot be cannot obtained be obtained from single from crystal single dicrystalffraction. diffraction. Therefore, Therefore, we consider we consider using using the selected the selected area areaelectron electron diffraction diffraction (SAED) (SAED) method method to analyze to analyze the sample, the butsample, the material but the is extremelymaterial is intolerant extremely to intolerantthe electron to beamthe electron and begins beam to and melt begins within to 2melt min within of irradiation 2 min of (see irradiation Figure S1). (see The Figure instability S1). The of instabilitymaterials bringsof materials many brings difficulties many to di thefficulties structural to the analysis. structural analysis.

TableTable 4. 4. TheThe unit unit cell cell parameters parameters and and crystal crystal system system of of BUCT-3 BUCT-3 molecular molecular sieves by XRD.

Unit CellUnit Parameters Cell Parameters BUCT-3 BUCT-3 a (Å) 8.9645 Standard deviationa of (Å) a 8.9645 0.0034 b (Å) 15.2727 StandardStandard deviation ofdeviation b of a 0.0034 0.0067 c (Å) 11.3907 Standard deviationb of (Å) c 15.2727 0.0034 α (◦) 90 Standardβ (◦) deviation of b 0.0067 93.858 γ (◦) 90 Crystal systemc (Å) 11.3907 Monoclinic

Standard deviation of c 0.0034 Although the speciﬁc structure model of the BUCT-3 is not available, some structural information of the molecular sieve can still be obtainedα (°) through FTIR,90 as shown in Figure9. Based on the detailed study of the infrared bands of zeolite by the KSF [39], some characteristic infrared bands of β (°) 93.858 silica-alumina molecular sieves were identiﬁed. The stretching vibration and bending modes of T-O-T 1 1 (T = Si or Al) can be found to have strongγ (°) absorption peaks at90 424 cm− and 970 cm− , respectively. The symmetrical stretching vibration peak of the silicon (aluminum) oxygen tetrahedron inside the Crystal system Monoclinic

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Although the specific structure model of the BUCT-3 is not available, some structural information of the molecular sieve can still be obtained through FTIR, as shown in Figure 9. Based on the detailed study of the infrared bands of zeolite by the KSF [39], some characteristic infrared bands Materialsof silica-alumina2020, 13, 5469 molecular sieves were identified. The stretching vibration and bending modes10 of of T- 14 O-T (T = Si or Al) can be found to have strong absorption peaks at 424 cm−1 and 970 cm−1, respectively. The symmetrical stretching vibration peak of the silicon (aluminum) oxygen tetrahedron inside the framework can be found at 650–750 cm 1. Furthermore, there are typical stretching vibration bands of framework can be found at 650–750 cm− −1. Furthermore, there are typical stretching vibration bands the SDA cation, tetrapropylammonium. The peaks near 2992 and 2987 cm 1 are characteristic of C-H of the SDA cation, tetrapropylammonium. The peaks near 2992 and 2987 cm− −1 are characteristic of C- stretching vibration and the peaks near 1483 and 1392 cm 1 are characteristic of N-C unsymmetrical H stretching vibration and the peaks near 1483 and 1392 cm− −1 are characteristic of N-C unsymmetrical bending vibration. Combined with the result of element analysis in Table5, it is believed that the SDA bending vibration. Combined with the result of element analysis in Table 5, it is believed that the cation was completely reserved in the framework, which means the structure directing property of SDA cation was completely reserved in the framework, which means the structure directing property TPA+ was not destroyed by the O-O bonds of the reactants. As mentioned before, O-O stretching of TPA+ was not destroyed by the O-O bonds of the reactants. As mentioned before, O-O stretching vibration can be found at the peak near 873 cm 1, and this can explain the poor thermostability of vibration can be found at the peak near 873 cm−−1, and this can explain the poor thermostability of BUCT-3 molecular sieves. On the other hand, the existence of O-O bonds in the framework may help BUCT-3 molecular sieves. On the other hand, the existence of O-O bonds in the framework may help to form a new structure of molecular sieves and extend the application of molecular sieves as well. to form a new structure of molecular sieves and extend the application of molecular sieves as well.

Figure 9. FTIR spectrum of BUCT-3: ( a) Full FTIR spectrum (b) ﬁngerprintfingerprint regionregion spectrum.spectrum.

Table 5.5. Element analysis of BUCT-3.

Weight Percentage (%) Atom Ratio Weight Percentage (%) Atom Ratio Element N C H O C/N BUCT-3 4.62 50.94 9.88 34.56 12.86 Materials 2020, 13, x FOR PEER REVIEW 11 of 14

Element N C H O C/N

Materials 2020, 13, 5469 BUCT-3 4.62 50.94 9.88 34.56 12.86 11 of 14

The full spectrum XPS of BUCT-3 and the high-resolution XPS of O are shown in Figure 10. From The full spectrum XPS of BUCT-3 and the high-resolution XPS of O are shown in Figure 10. From the the full spectrum, we can see the main elements such as Si, Al, O, C, and N in the template. From the full spectrum, we can see the main elements such as Si, Al, O, C, and N in the template. From the peak peak fitting spectrum of O, it can be known that in addition to the skeleton O with binding energy at ﬁtting spectrum of O, it can be known that in addition to the skeleton O with binding energy at 532 eV − 2− 532 eV and the surface silanol OH at 531 eV, there is a most2 typical O2 peak at 530 eV. Through and the surface silanol OH− at 531 eV, there is a most typical O2 − peak at 530 eV. Through quantitative quantitative analysis and calculation, we have obtained 6.4% O22− of the total O 1s. Since BUCT-3 is analysis and calculation, we have obtained 6.4% O 2 of the total O 1s. Since BUCT-3 is prepared from prepared from SiAl-2, only a small part of the 23%2 oxygen− in SiAl-2 enters the final structure. This is SiAl-2, only a small part of the 23% oxygen in SiAl-2 enters the ﬁnal structure. This is because most because most of the O-O is decomposed by heating during the crystallization process. Combined of the O-O is decomposed by heating during the crystallization process. Combined with the above with the above infrared analysis, the conclusion that the peroxide bonds in the aluminosilica infrared analysis, the conclusion that the peroxide bonds in the aluminosilica perhydrate hydrogel are perhydrate hydrogel are retained in the framework of the product has been strongly proved. retained in the framework of the product has been strongly proved.

Figure 10. XPS spectra ofof BUCT-3:BUCT-3: ((aa)) the the full full XPS XPS survey survey spectra, spectra, (b ()b high-resolution) high-resolution XPS XPS spectra spectra of O.of O. In situ variable temperature XRD analysis reveals the thermostability of the as-synthesized BUCT-3 (FigureIn 11situ). BUCT-3variable molecular temperature sieves XRD started analysis to collapse reveals at the 140 thermostability◦C, which can be of clearly the as-synthesized seen from the BUCT-3signiﬁcant (Figure decrease 11). of BUCT-3 the peaks’ molecular intensities. sieves It isstarted probably to collapse because at the 140 O-O °C, bonds which in can the be framework clearly seen of fromBUCT-3 the brokesignificant during decrease the heating of the process. peaks’ Hence,intensities. the optimalIt is probably calcination because condition the O-O for bonds BUCT-3 in the to frameworkremove templates of BUCT-3 is at broke a lower during temperature, the heating like proc 120 ◦ess.C, in Hence, the presence the optimal of ozone calcination and this condition work is stillfor BUCT-3in progress. to remove templates is at a lower temperature, like 120 °C, in the presence of ozone and this work is stillMaterials in progress. 2020, 13, x FOR PEER REVIEW 12 of 14 Based on the above conclusions and the hydrogen-bond induced crystallization mechanism of zeolites [40], we can reasonably speculate on the crystallization process of zeolites with the new structure. In the early stage of the reaction, aluminosilica perhydrate hydrogel first depolymerizes into a large number of monomeric and dimeric species with hydrogen-bond (H-O-O). Then, these species condense around TPA+ through the hydrogen bonds on its surface. These hydrogen bonds from the [(HO)3SiOSi(OH)3]-H2O2 [31] or SiO−…HO-Si [40] serve as the connectors to bridge the silicates together and fuse themselves to form the long-range periodic framework. During the condensation process, the peroxide bond (O-O) of hydrogen peroxide is finally incorporated into the framework.

Figure 11. In situ variable temperature XRD patterns of BUCT-3. Figure 11. In situ variable temperature XRD patterns of BUCT-3. 4. Conclusions In this paper, two kinds of new silicon and aluminum sources obtained by gel-sol method are aluminosilica perhydrate hydrogels and both of them have amorphous phase with nano-sphere morphology. FTIR analysis was used to confirm the existence of O-O bonds in the silicon and aluminum source, SiAl-1 and SiAl-2. Pure phase molecular sieves, BUCT-3, was hydrothermally synthesized by using TPAOH as SDA under mild reacting condition. The key to synthesize this new aluminosilicate is using aluminosilica perhydrate hydrogel with relatively high O-O bond contents, SiAl-2, as the silicon and aluminum source. The cell parameters of BUCT-3 determined by XRD were a = 8.9645 Å, b = 15.2727 Å, c = 11.3907 Å, α = 90°, β = 93.858°, γ = 90°. Since the O-O bonds remained in the structure of BUCT-3, its thermostability is not good. Despite the advances described in the structure elucidation of new materials, there are occasions where the achievement of the real structure of unstable materials is still a complex problem. Therefore, it is necessary to develop a new characterization technique or preparation procedure to solve this extremely unstable structure, so that the detailed structure information of the molecular sieve with peroxide bond in the framework can be obtained. Nevertheless, BUCT-3 molecular sieves may have potential application as a bleaching and oxidizing agent. Besides, this new synthesis method of H2O2 treatment provides an effective strategy for the design of new molecular sieves.

Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: the selected area electron diffraction (SAED) of BUCT-3: (a) Irradiate for 1 minute (b) Irradiate for 2 minutes (c) Irradiate for 2.5 minutes.

Author Contributions: Conceptualization, K.J.; Formal analysis, H.M.; Investigation, H.M. and K.J.; Methodology, H.M.; Supervision, X.X. and J.S; Writing—original draft, H.M. and K.J.; Writing—review & editing, X.X. and J.S. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by [The Shanghai Key Laboratory of Green Chemistry and Chemical Processes] grant number [H2016107].

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Rabo, J.A.; Kasai, P.H. Caging and electrolytic phenomena in zeolites. Prog. Solid State Chem. 1975, 9, 1–19.

Materials 2020, 13, 5469 12 of 14

Based on the above conclusions and the hydrogen-bond induced crystallization mechanism of zeolites [40], we can reasonably speculate on the crystallization process of zeolites with the new structure. In the early stage of the reaction, aluminosilica perhydrate hydrogel ﬁrst depolymerizes into a large number of monomeric and dimeric species with hydrogen-bond (H-O-O). Then, these species condense around TPA+ through the hydrogen bonds on its surface. These hydrogen bonds from the ... [(HO)3SiOSi(OH)3]-H2O2 [31] or SiO− HO-Si [40] serve as the connectors to bridge the silicates together and fuse themselves to form the long-range periodic framework. During the condensation process, the peroxide bond (O-O) of hydrogen peroxide is ﬁnally incorporated into the framework.

4. Conclusions In this paper, two kinds of new silicon and aluminum sources obtained by gel-sol method are aluminosilica perhydrate hydrogels and both of them have amorphous phase with nano-sphere morphology. FTIR analysis was used to conﬁrm the existence of O-O bonds in the silicon and aluminum source, SiAl-1 and SiAl-2. Pure phase molecular sieves, BUCT-3, was hydrothermally synthesized by using TPAOH as SDA under mild reacting condition. The key to synthesize this new aluminosilicate is using aluminosilica perhydrate hydrogel with relatively high O-O bond contents, SiAl-2, as the silicon and aluminum source. The cell parameters of BUCT-3 determined by XRD were a = 8.9645 Å, b = 15.2727 Å, c = 11.3907 Å, α = 90◦, β = 93.858◦, γ = 90◦. Since the O-O bonds remained in the structure of BUCT-3, its thermostability is not good. Despite the advances described in the structure elucidation of new materials, there are occasions where the achievement of the real structure of unstable materials is still a complex problem. Therefore, it is necessary to develop a new characterization technique or preparation procedure to solve this extremely unstable structure, so that the detailed structure information of the molecular sieve with peroxide bond in the framework can be obtained. Nevertheless, BUCT-3 molecular sieves may have potential application as a bleaching and oxidizing agent. Besides, this new synthesis method of H2O2 treatment provides an eﬀective strategy for the design of new molecular sieves.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/23/5469/s1, Figure S1: the selected area electron diffraction (SAED) of BUCT-3: (a) Irradiate for 1 minute (b) Irradiate for 2 minutes (c) Irradiate for 2.5 minutes. Author Contributions: Conceptualization, K.J.; Formal analysis, H.M.; Investigation, H.M. and K.J.; Methodology, H.M.; Supervision, X.X. and J.S; Writing—original draft, H.M. and K.J.; Writing—review & editing, X.X. and J.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by [The Shanghai Key Laboratory of Green Chemistry and Chemical Processes] grant number [H2016107]. Conflicts of Interest: The authors declare no conflict of interest.

References

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