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crystals Article Synthesis, Characterization and Catalytic Performance of Well-Ordered Crystalline Heteroatom Mesoporous MCM-41 Jing Qin 1,2, *, Baoshan Li 1, * and Dongpeng Yan 1,3, * 1 2 3 * State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of environmental protection and biological pharmacy, Beijing Industrial Technician College, Beijing 100023, China College of Chemistry, Beijing Normal University, Beijing 100875, China Correspondence: [email protected] (J.Q.); [email protected] (B.L.); [email protected] (D.Y.); Tel.: +86-10-6738-3433 (J.Q.) Academic Editors: Helmut Cölfen and Mei Pan Received: 27 February 2017; Accepted: 17 March 2017; Published: 23 March 2017 Abstract: Mesoporous heteroatom molecular sieve MCM-41 bulk crystals with the crystalline phase were synthesized via a one-step hydrothermal method using an ionic complex as template. The ionic complex template was formed by interaction between cetyltrimethylammonium ions and metal complex ion [M(EDTA)]2− (M = Co or Ni)]. The materials were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, N2 adsorption-desorption isotherms, and X-ray absorption fine structure spectroscopy. The results showed that the materials possess a highly-ordered mesoporous structure with a crystalline phase and possess highly uniform ordered arrangement channels. The structure is in the vertical cross directions with a crystalline size of about 12 µm and high specific surface areas. The metal atoms were incorporated into the zeolite frameworks in the form of octahedral coordinate and have a uniform distribution in the materials. The amount of metal complexes formed by metal ion and EDTA is an essential factor for the formation of the vertical cross structure. Compared to Si-MCM-41, the samples exhibited better conversion and higher selectivity for cumene cracking. Keywords: bulk crystal mesoporous MCM-41; heteroatom molecular sieves; direct hydrothermal method 1. Introduction By virtue of their high thermal and hydrothermal stability, microporous zeolites are widely used in industries as heterogeneous catalysts, particularly as solid acid catalysts in the fields of oil refining and petrochemistry [1]. Although the micropores of zeolites have often been described as having excellent potential for chemical functions, their pore size often limits the reaction rate and thus have an adverse effect on practical chemical processes [2–4]. In 1992, in an attempt to overcome this problem, Mobil Oil Corporation synthesized a family of mesoporous solids named M41s [5] which possess an ordered arrangement of larger pores with diameters of 2–10 nm. However, the use of these materials as catalysts is hampered by their relatively low thermal stability and weak acid strength [6,7]. Therefore, it would be of great interest to synthesize a new type of material combining the advantages of both mesoporous molecular sieves and zeolites [8,9]. Only a few studies have been published to date; for example, Karlsson et al. prepared MFI/MCM-41 composites using two templates (C6 H13 (CH3 )3 NBr and C14 H29 (CH3 )3 NBr), and optimized the template concentrations and reaction temperature [10]. In previous studies, we have reported the synthesis and characterization of composite molecular sieves Crystals 2017, 7, 89; doi:10.3390/cryst7040089 www.mdpi.com/journal/crystals Crystals 2017, 7, 89 2 of 11 M1 -MFI/M2 -MCM-41 (M1 , M2 = Ni, Co) and their catalytic properties for the hydrocracking of residual oil [11] and the preparation of micro–mesoporous FeCo-MFI/MCM-41 composites which gave a clear improvement in the hydrocracking of residual oil [12]. However, the resulting materials are composites of zeolite crystallites embedded in a disordered mesoporous matrix. Kaliaguine et al. described a general method for the production of a new type of material with semi-crystalline zeolite mesopore walls [13]. Other attempts to crystallize the mesopore walls of mesostructure materials have resulted in the formation of materials with increased thermal stability and catalytic activity. For example, ITQ zeolites having a partially crystalline bimodal pore system with a combination of micropores and mesopores have been synthesized by delaminating the layered zeolite precursors MCM-22 and ferrierite [14]. However, none of these techniques yields a regular distribution of mesopores, let alone an ideal channel system of mesopores structurally connected with the regular micropores of the zeolite. Jiang et al. reported the synthesis of zeolite ITQ-43, which features a structurally hierarchical system of connected mesopores and micropores. Although high-resolution transmission electron microscopy (HRTEM) did not show the mesopore features, evidence for the presence of mesopores was provided by automated electron diffraction tomography [15]. W. Chaikittisilp recently demonstrated a new approach for the construction of hierarchically organized zeolites by sequential intergrowth; for the first time, a complex and unusual morphology was observed for MFI zeolites [16]. In this paper, M-MCM-41 bulk crystal heteroatom molecular sieves with crystalline phase have been synthesized using a direct hydrothermal method. The materials possess a crystalline phase and highly uniform ordered arrangement channels. This is a new type of crystalline mesoporous molecular sieve. Three-dimensional crystal mesoporous MCM-41 was obtained in typical synthesis conditions. 2. Results The samples were labeled as Cry-Ni and Cry-Co, and the metal heteroatom contents of the samples measured by inductively coupled plasma (ICP) are listed in Table 1. Table 1. Textural properties of the samples. ICP: inductively coupled plasma. Sample M Contents wt. % (by ICP) SBET (m2 ·g−1 ) Pore Volume (cm3 ·g−1 ) Pore Diameter (nm) Cry-Ni Cry-Co 7.88 6.77 273 425 1.12 0.97 5.43 3.51 Low angle XRD patterns of the samples are presented in Figure 1. All samples show an intense broad diffraction peak at very low 2θ angle of 0.7◦ –1.7◦ , with an overlapping broader feature at 2◦ –7◦ , which can be respectively attributed to the (1 0 0) diffraction peak and the overlapping pair of (1 1 0) and (2 0 0) diffraction peaks, indicating that the samples have good structural order and the obvious mesoporous structure of MCM-41 [17]. The particle size and shape of the samples were investigated using SEM, and the images are shown in Figure 2a,b. The results show that the samples have well-defined morphologies with a crystal structure (corresponding to the XRD results), and the crystalline sizes are about 12 µm with a uniform distribution. It can be seen from the images that the two samples have good dispersion. In addition, the materials possess a structure in the vertical cross direction. This is determined presumably by the structure of the metal complex. The SEM-mappings of the samples are also shown in Figure 2a*,b*, which display the composition and distribution of elements in the samples. It is clear that all samples (Cry-Ni and Cry-Co) have uniform distribution of elements, and the metal atoms (Ni and Co) were highly dispersed in the materials. It can be seen from Figure 2e,f that the two samples have good dispersion. After grinding with a ball mill, the samples were tested by HRTEM, and the results are shown in Figure 2c,d. The images indicate that the materials exhibit a uniform pore size with a highly-ordered pore structure and the characteristic long-range ordered symmetry of a mesophase. The ordered mesopores of the crystals can be easily observed in the HRTEM images. Crystals 2017, 7, 89 3 of 10 with a highly-ordered pore structure and the characteristic long-range ordered symmetry of a Crystals 2017,ordered 7, 89 3 ofimages. 11 mesophase. The mesopores of the crystals can be easily observed in the HRTEM Crystals 2017, 7, 89 3 of 10 with a highly-ordered pore structure and the characteristic long-range ordered symmetry of a mesophase. The ordered mesopores of the crystals can be easily observed in the HRTEM images. Figure 1. XRD patterns patterns of the samples: (a) Cry-Ni; Cry-Ni; (b) (b) Cry-Co. Figure 1. XRD the samples: samples: (a) Cry-Co. Figure 1. XRD patterns ofofthe (a) Cry-Ni; (b) Cry-Co. Figure 2. Cont. Crystals 2017, 7, 89 4 of 11 Crystals 2017, 7, 89 4 of 10 Crystals 2017, 7, 89 4 of 10 Figure 2. SEM images, SEM-mappings, and high-resolution TEM (HRTEM) images of the samples: Figure 2. SEM images, SEM-mappings, and high-resolution TEM (HRTEM) images of the samples: Cry-Ni (a,a*,c,e) and Cry-Co (b,b*,d,f). Cry-Ni (a,a*,c,e) and Cry-Co (b,b*,d,f). and high-resolution TEM (HRTEM) images of the samples: Figure 2. SEM images, SEM-mappings, Cry-Ni (a,a*,c,e) and Cry-Co (b,b*,d,f). The handbook of X-ray photoelectron spectroscopy (XPS) showed the binding energy of NiO and CoO. The binding energy peak of NiOspectroscopy was between 854 eV and 855 eV. peak ofenergy CoO and The handbook of X-ray photoelectron (XPS) showed theThe binding of NiO The handbook of eV. X-ray photoelectron spectroscopy (XPS) showed the binding energyand of NiO Co 3 O 4 centered at 780 Figure 3 shows the XPS spectra of Cry-Ni and Cry-Co samples, the and CoO. The binding energy peak of NiO was between 854 eV and 855 eV. The peak of CoO and and CoO. binding energyNi peak of NiO was between 854 eVAs and 855 eV. peak CoO is and are The the de-convoluted 2p3/2 andXPS Co spectra 2p3/2 spectra. shown in The Figure 3a,ofthere ainsets Co3 Oinsets at 780 eV. Figure 3 shows the of Cry-Ni and Cry-Co samples, and the 4 centered Co 3O4 centered at 780 eV. Figure 3 shows the XPS spectra of Cry-Ni and Cry-Co samples, and the predominant peak centered at 855.95 eV [18], indicating the strong interaction between nickel atom are the de-convoluted Ni 2p3/2 and Co 2p3/2 spectra. shown Figurein3a,Figure there 3a, is athere predominant insets thebase, de-convoluted Co 2p3/2 As spectra. Asin shown is a and theare silica suggesting Ni the2p3/2 nickeland atoms incorporating into the silica framework [19], which peak predominant centered at 855.95 eV [18],atindicating the indicating strong interaction between nickel atom andatom the silica peak centered 855.95 eV [18], the strong interaction between nickel corroborates the previous results; a peak centered at 781.55 eV in the Figure 3b is the characteristic base,peak suggesting the nickel atoms that incorporating into the silica framework [19], which corroborates and the silica base, suggesting nickel atoms incorporating into the silica framework [19], which of Co 2p2/3 [20], indicatingthe cobalt atoms were also incorporating into the silica framework. corroborates the previous results; a peak centered eV in the Figure 3b is the characteristic the previous results; a peak centered at 781.55 eV at in781.55 the Figure 3b is the characteristic peak of Co Co 2p2/3 [20], that cobalt were also incorporating into the silica framework. 2p2/3peak [20],ofindicating thatindicating cobalt atoms wereatoms also incorporating into the silica framework. Figure 3. X-ray photoelectron spectroscopy (XPS) spectra of the samples: (a) Cry-Ni; (b) Cry-Co. Figure 4 3. illustrates the X-ray absorption (XPS) near-edge structure (XANES) spectra ofCry-Co. Cry-Ms. The Figure 3. X-ray photoelectron spectroscopy ofthe thesamples: samples: Cry-Ni; (b) Cry-Co. Figure X-ray photoelectron spectroscopy (XPS) spectra spectra of (a)(a) Cry-Ni; (b) corresponding spectra for metal oxides are given for comparison. The XANES pre-edge peak is Figureto4the illustrates the X-ray absorption near-edge (XANES) spectra of Cry-Ms. The attributed 1s-3d the dipolar forbidden transition [21]. structure Principally, this(XANES) 1s-3d forbidden transition Figure 4 illustrates X-ray absorption near-edge structure spectra of Cry-Ms. corresponding spectra for metal oxides are given for comparison. The XANES pre-edge peak is gains additional intensity when the metal center is in a non-centrally symmetric environment The corresponding spectra for metal oxides are given forPrincipally, comparison. The XANES pre-edge peak is attributed to the 1s-3d dipolar forbidden transition [21]. this 1s-3d forbidden transition through mixing of 3d and 4p orbitals caused by the breakdown of inversion symmetry due to the attributed the 1s-3d dipolarwhen forbidden transition thissymmetric 1s-3d forbidden transition gains to additional intensity the metal center [21]. is in Principally, a of non-centrally environment structure distortion. Figure 4a illustrates the XANES spectra NiO and Cry-Ni. The nickel atoms in gainsthrough additional intensity when metalcaused centerby is the in abreakdown non-centrally symmetric environment mixing of 3d and 4pthe orbitals of inversion symmetry due to through the structure distortion. Figurecaused 4a illustrates thebreakdown XANES spectra of NiO andsymmetry Cry-Ni. The nickel atoms in mixing of 3d and 4p orbitals by the of inversion due to the structure distortion. Figure 4a illustrates the XANES spectra of NiO and Cry-Ni. The nickel atoms in Cry-Ni have similar pre-edge features to that of NiO, which indicates nickel exists as mainly Ni2+ in the Crystals 2017, 7, 89 5 of 11 material. Figure Crystals 2017, 7, 4b 89 displays the XANES spectra of CoO, Co3 O4 , and Cry-Co. The cobalt 5atoms of 10 in Cry-Co have similar features to CoO, suggesting that Co2+ exists in the sample. Note that the peaks Cry-Nishifted have similar pre-edge features to that of NiO,samples which indicates exists as mainly Ni2+ shifts. in are slightly to a longer distance than standard becausenickel of the scattering phase the material. Figure 4b displays the XANES spectra of CoO, Co3O4, and Cry-Co. The cobalt atoms in Extended X-ray absorption fine structure (EXAFS) spectroscopy [22] is one of the most powerful Cry-Co have similar features to CoO, suggesting that Co2+ exists in the sample. Note that the peaks structural characterization methods, especially for interface-confined effects, due to its sensitivity to are slightly shifted to a longer distance than standard samples because of the scattering phase shifts. the three-dimensional short-range order (typically 0–8 Å) and chemical state, including near-neighbor Extended X-ray absorption fine structure (EXAFS) spectroscopy [22] is one of the most powerful 2 species, distance (R), coordination number (N), and disorder in bondeffects, distance 4a* shows structural characterization methods, especially for interface-confined due(σto).itsFigure sensitivity to 3 the K the -weighted spectra of Cry-Ni and order NiO, which are0–8 the Å) Fourier transformed of Ni K-edge three-dimensional short-range (typically and chemical state,spectra including nearEXAFS spectraspecies, in the range of k(R), = 2–6 Å. The peaks correspond to the distance a 2Ni atom and neighbor distance coordination number (N), and disorder in bond between distance (σ ). Figure 4a* shows the K3-weighted spectra Cry-Ni and NiO,reference which arematerial), the Fourier transformed spectra(– of 1 ) 5 its neighboring atoms. In the curve of of NiO (a standard there are five peaks Ni K-edge EXAFS spectra in the range of k = 2–6 Å. The peaks correspond to the distance between a 2 Ni-O , 3 Ni-Ni Ni-Ni-O, Ni-O-Ni-O, Ni-O , 4 and Ni-Ni 5 that can be ascribed to Ni-O, Ni-Ni , Ni atom and respectively. its neighboringItatoms. In the of NiOof (a Cry-Ni standardisreference material), there are five scattering paths, is clear thatcurve the curve different from the curve of NiO, peaks (①–⑤) that can be ascribed to Ni-O, Ni-Ni ②, Ni-O ③, Ni-Ni Ni-Ni-O, Ni-O-Ni-O, Ni-O ④, 1 and 2 of Cry-Ni are similar to the peaks 1 and 2 of NiO; a new peak although the two peaks and Ni-Ni ⑤ scattering paths, respectively. It is clear that the curve of Cry-Ni is different from the 1 and 2 of Cry-Ni presents, which is originated in the silicon atoms. The shape between peaks curve of NiO, although the two peaks ① and ② of Cry-Ni are similar to the peaks ① and ② of 3 , 4 and 5 are also different from that of NiO. The results show that the , and intensity of peaks NiO; a new peak between peaks ① and ② of Cry-Ni presents, which is originated in the silicon structure forms Ni atoms in Cry-Ni are different from in NiO. Similar results were obtained atoms. The of shape and intensity of peaks ③, ④, and ⑤ are also different from that of NiO. The in the Cry-Co 4b* presents Fourier transforms of k3 -weighted Co K-edge EXAFS results sample. show thatFigure the structure forms of the Ni atoms in Cry-Ni are different from in NiO. Similar results 3-weighted spectra of Cry-Co, Co3 O (standard reference materials). The curve of Cry-Co also presents were obtainedCoO, in theand Cry-Co sample. Figure 4b* presents the Fourier transforms of k Co 4 K-edge ofare Cry-Co, CoO, andthe Co3peaks O4 (standard reference materials). curve Cry1 EXAFS 2spectra 1 and 2 of the two peaks and , which similar with curve ofThe CoO, butofare much Co also presents two of peaks ①4 .and which are similar with the peaks ① the curve of 1 and 2and different from the curve Co3 O The②, small peak between peaks in ② theofcurve of Cry-Co but are different from the of Co 3O4. The small peak between peaks ① and ② in is alsoCoO, induced bymuch the silicon atoms. Thecurve results are useful for estimation of the crystallinity of the the curve of Cry-Co is also induced by the silicon atoms. The results are useful for estimation of the samples, and the analytic results are listed in Table 2. crystallinity of the samples, and the analytic results are listed in Table 2. The XAFS results indicated that each metal center has a coordination number six with The XAFS results indicated that each metal center has a coordination number six with an an octahedral coordination atoms.The Thebonds bonds M-O-Si present in the Cry-Ms octahedral coordinationwith with the the surrounding surrounding atoms. of of M-O-Si present in the Cry-Ms samples, and this suggests that the metal atoms are incorporated in the silica framework rather samples, and this suggests that the metal atoms are incorporated in the silica framework rather thanthan existing as a separate oxide phase. existing as a separate oxide phase. Figure 4. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure Figure 4. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of samples: (a,a*) Cry-Ni; (b,b*) Cry-Co. (EXAFS) of samples: (a,a*) Cry-Ni; (b,b*) Cry-Co. Sample Coordination Type N R (Å) σ2 (Å2) Ni–O 6.0 ± 0.946 2.053 ± 0.031 0.00597 ± 0.0008 Ni–Ni 12.0 ± 0.556 3.091 ± 0.143 0.00833 ± 0.0008 Ni–O 6 2.084 12 2.948 Cry-Ni Crystals 2017, 7, 89 NiO Cry-Co Ni–Ni Co–O Sample CoO Cry-Ni Co3O4 NiO Cry-Co Table 2. EXAFS fitting results for samples. Co–Co Coordination Type Co–ONi–O Ni–Ni Co–Co Ni–O Co–O Ni–Ni Co–Co Co–O Co–Co 6 of 11 6.0 ± 0.803 2.060 ± 0.073 0.00954 ± 0.0008 12.0 ±N0.398 3.116 ± 0.099 R (Å) 0.00974 σ2 (Å2 ) ± 0.0008 6.0 ± 6 0.946 12.0 ± 0.556 2.053 ± 0.031 2.133 3.091 ± 0.143 6 412 1.917 2.948 6 0.803 6.0 ± 12.0 ± 0.398 1.942 2.060 ± 0.073 3.116 ± 0.099 12 3.017 0.00597 ± 0.0008 0.00833 ± 0.0008 2.084 0.00954 ± 0.0008 0.00974 ± 0.0008 The N2 adsorption-desorption isotherm method was employed to investigate the pore structur Co–O 6 2.133 CoO materials. The isotherms of the Co–Co samples obtained at −196 3.017 °C are illustrated in Figure 5. It show 12 Co–O 4 at the isotherms of the samples are type IV [23] and possess 1.917 type H1 hysteresis loops. It is indicate Co3 O 4 Co–Co 6 1.942 at the samples are typical mesoporous materials, exhibiting a sharp characteristic of capillar ondensation at intermediate partial pressures (0.3 <was P/P 0 < 0.5). In the relative pressure range 0.90 The N2 adsorption-desorption isotherm method employed to investigate the pore structure of ◦ materials. isothermssharp of the samples at −196 C illustrated in Figure 5. It shows that the 99, all samples showThe another withobtained the increase ofarethe nitrogen adsorption volume, resultin isotherms of the samples are type IV [23] and possess type H1 hysteresis loops. It is indicated that the om filling thesamples macropores formed by interparticle spaces and untransformed amorphous silica. Th are typical mesoporous materials, exhibiting a sharp characteristic of capillary condensation xtural properties of samples listed Table 1.InThe pore diameters the two samples have littl at intermediate partial are pressures (0.3 in < P/P the relative pressure range of 0.90–0.99, all samples 0 < 0.5). show another sharp with the increase of the nitrogen adsorption volume, resulting from filling fference from each other, due to the different micelle diameter of [(C4H9)4N][M(EDTA)] caused b the macropores formed by interparticle spaces and untransformed amorphous silica. The textural 2+ 2+, which also influences the specific surface are e difference properties radius of heteroatoms of samples are listed inNi Table and 1. The Co pore diameters of the two samples have little difference from each to the different micelle diameter of [(C4 H9 )4 N][M(EDTA)] caused by the difference ore volume, and the other, poredue diameter. radius of heteroatoms Ni2+ and Co2+ , which also influences the specific surface area, pore volume, and the pore diameter. Figure 5. N2 adsorption–desorption isotherms of the samples (inset: pore size distribution of the samples: (a) Cry-Ni; (b) Cry-Co. isotherms of the samples (inset: pore size distribution of the Figure 5. N2 adsorption–desorption samples: (a) Cry-Ni; (b) Cry-Co. The 0.5 g samples of Cry-Ni and Cry-Co were placed in 50 mL 1 mol/L NaOH solution, stirring at room temperature for 2 h, then samples were filtered and dried. The morphological changes The 0.5 g samples of Cry-Ni andunder Cry-Co were placedand in 50 1 mol/L NaOH solution, stirrin of the samples were observed electron microscope, the mL morphology of the samples is shown infor Figure The crystal structure of filtered the samples treated by sodium hydroxide solutionchanges of th room temperature 2 h,6.then samples were and dried. The morphological is shown. The samples covering the surface of the oxide dissolved by the lye treatment, and the mples were observed under electron and the morphology the samples is shown i crystal exhibits relatively completemicroscope, form. Mesoporous molecular sieve material byof traditional alkali treatment,structure hole wall collapse phenomenon willtreated appear. Inby contrast, the crystal structure ofsolution metal atomsis shown. Th gure 6. The crystal of the samples sodium hydroxide in the crystalline molecular sieve improves the alkali resistance of the molecular sieve. mples covering the surface of the oxide dissolved by the lye treatment, and the crystal exhibit latively complete form. Mesoporous molecular sieve material by traditional alkali treatment, hol all collapse phenomenon will appear. In contrast, the crystal structure of metal atoms in th ystalline molecular sieve improves the alkali resistance of the molecular sieve. Crystals 2017, 7, 89 7 of 10 Crystals 2017, 7, 89 7 of 11 rystals 2017, 7, 89 7 of Figure 6. SEM micrographs of the samples treated with NaOH solution: (a) Cry-Ni; (b) Cry-Co. According to the above results, a possible mechanism was proposed that is revealed as Scheme 1. There are two nitrogen atoms andoffour carboxyl oxygen atoms (marked as A, C, D, respectively) 6. SEM micrographs the samples treated with NaOH solution: (a) Cry-Ni; (b) B, Cry-Co. Figure 6. SEMFigure micrographs of the samples treated with NaOH solution: (a) Cry-Ni; (b) Cry-Co. in the EDTA molecule which can coordinate with the metal ion to form the complex ion [M(EDAT)]2− According to the above a possible mechanism was proposed is revealed Scheme with an octahedral structure. Inresults, the chelate ion, the carboxyl oxygenthat atoms A, B,asand two1.nitrogen According to the above results, a possible mechanism was proposed that is revealed as Schem There are two nitrogen atoms and four carboxyl oxygen atoms (marked as A, B, C, D, respectively) in vertical atoms are at a flat surface, and the other two carboxyl oxygen atoms C and D are in the 2 − the nitrogen EDTA molecule which can coordinate with theoxygen metal ion to form the complex ion Theredirection are two and fourthe carboxyl atoms as[M(EDAT)] A, B, C,was D, respectivel but not at aatoms flat surface with two nitrogen atoms. As (marked the complex solution added with an octahedral structure. In the chelate ion, the carboxyl oxygen atoms A, B, and two nitrogen 2− [M(EDAT)] + n the EDTA which can coordinate with thesolution, metal ion to form the complex ion into themolecule cetyltrimethyl ammonium ionic atoms are at a flat surface, andbromide the other (CTAB) two carboxyl oxygenthe atoms C associate and D are [M(EDAT)] in the vertical (CTA )2 the special structure (Scheme micelle suspension wastwo mixed direction but not at amicelle flat withbe theformed two nitrogen atoms.1). AsAs thethe complex solution was with anand octahedral structure. In surface thewould chelate ion, the carboxyl oxygen atoms A, B,added and nitroge 2− (CTA+ ) 4− 4− into the cetyltrimethyl ammonium bromide (CTAB) solution, the ionic associate [M(EDAT)] withat thea solution of SiO4 and formed SiO2 and of sodium hydroxide, ions toms are flat surface, the by other twoplenty carboxyl oxygen atoms SiO C 4and Dwould are2 inrapidly the vertic and the special structure micelle would be formed (Scheme 1). As the micelle suspension was mixed hydrolyze and polymerize around the micelle which possessed a structure in the vertical cross irection but not flat surface the nitrogen As the complex was adde with at theasolution of SiO4 4−with formed by two SiO2 and plenty ofatoms. sodium hydroxide, SiO4 4 − ionssolution would direction. Additionally, the crystalline mesoporous zeolite precursors would grow along the vertical 2− (CTA+ rapidly hydrolyze and polymerize around(CTAB) the micellesolution, which possessed aionic structure in the vertical cross nto thecross cetyltrimethyl ammonium bromide thethe associate [M(EDAT)] 2− direction in the following hydrothermal process. Because structure of the [M(EDAT)] direction. Additionally, the crystalline mesoporous zeolite precursors would grow along the vertical nd thecomplex specialcross micelle be formed (Scheme 1).theAs the micelle suspension was 2 − same isstructure not the same in following thewould vertical cross direction, micelle structure would not be the in mixe direction in the hydrothermal process.the Because structure of the [M(EDAT)] 4− is not the same in by the vertical cross direction, micelle structure not be4structures theions samewould with thethe solution of SiO 44− formed SiOfactor 2 and plenty ofthesodium hydroxide, SiO two complex directions; this is the main causing the sample to havewould different in therapid in the two directions; this is the main factor causing the sample to have different structures in the different (Figure 2). A certain amountwhich of the complexes by metalin ionthe andvertical EDTA cro ydrolyze and directions polymerize around the micelle possessedformed a structure different directions (Figure 2). A certain amount of the complexes formed2−by metal ion and EDTA was wasAdditionally, essential in thethe formation of themesoporous structure. Enough [M(EDTA)] could ensure the formation 2 precursors − could ensure irection. crystalline zeolite would grow along theofvertic essential in the formation of the structure. Enough [M(EDTA)] the formation of single single structural micelleswith with the vertical structure, which is the prerequisite of the formation structural thehydrothermal vertical cross cross structure, which isBecause the prerequisite the formation the ross direction in themicelles following process. the ofstructure ofof the [M(EDAT)] of the crystalline zeoliteprecursors. precursors. crystalline zeolite omplex is not the same in the vertical cross direction, the micelle structure would not be the same he two directions; this is the main factor causing the sample to have different structures in th ifferent directions (Figure 2). A certain amount of the complexes formed by metal ion and EDT was essential in the formation of the structure. Enough [M(EDTA)]2− could ensure the formation ngle structural micelles with the vertical cross structure, which is the prerequisite of the formatio f the crystalline zeolite precursors. Scheme 1. The supposed formation formation mechanism of Cry-M. Scheme 1. The supposed mechanism of Cry-M. The catalytic performance of the crystalline molecular sieves for cumene cracking was investigated. The device diagram is shown in Figure 7. Before the reaction, the samples were activated at 300 °C and nitrogen atmosphere for one hour. Reaction conditions were as follows: temperature, 30 °C; nitrogen atmosphere; mass ratio of raw material/catalyst, 100. After a certain reaction time, the Crystals 2017, 7, 89 8 of 11 The catalytic performance of the crystalline molecular sieves for cumene cracking was investigated. The device diagram is shown in Figure 7. Before the reaction, the samples were activated at 300 ◦ C stals 2017, 7, 89 and nitrogen atmosphere for one hour. Reaction conditions were as follows: temperature, 30 ◦ C; 8 of 1 nitrogen atmosphere; mass ratio of raw material/catalyst, 100. After a certain reaction time, the results are shown in Figure 8.decreased The first peak is the20 cumene peak, the second peak for small molecule mples were not significantly after times, indicating thatisthe samples’ recycling rate hydrocarbon, the third peak is attributed to benzene homologues. It can be seen from the figure e higher, andthat themetal highest conversion of 54.5% anda catalytic 44.03%effect was cumene cracking atoms in the crystallinerate molecular sieves have onreached. the crackingThe of cumene. Figureacid 8a*,b* catalytic for the catalytic conversion curves, the silicon catalytic efficiency of Cry-Ni and Cry-Co samples action is a typical reaction. The pure Si-MCM-41 molecular sieves have almos were not significantly decreased after 20 times, indicating that the samples’ recycling rates are higher, acid, so there is no catalytic function of the reaction. Comparing to Si-MCM-41, the sample and the highest conversion rate of 54.5% and 44.03% was reached. The cumene cracking reaction is hibited better conversion higher selectivity for molecular cumenesieves cracking. The better catalyti a typical acid catalyticand reaction. The pure silicon Si-MCM-41 have almost no acid, so there is no catalytic of the reaction. Comparing to Si-MCM-41, the appropriate samples exhibitedpore betterdiameter, and rformance is attributed to thefunction samples possessing uniform channels, Crystals 2017, 7, 89 8 of 10 conversion and higher selectivity for cumene cracking. The better catalytic performance is attributed ive sites caused bysamples the introduction of Ni and Co atomspore intodiameter, the framework ofcaused molecular to the possessing uniform channels, appropriate and active sites by the sieves. samples were not significantly decreased after 20 times, indicating that the samples’ recycling rates introduction of and Ni and Co atoms into therate framework of molecular sieves. are higher, the highest conversion of 54.5% and 44.03% was reached. The cumene cracking reaction is a typical acid catalytic reaction. The pure silicon Si-MCM-41 molecular sieves have almost no acid, so there is no catalytic function of the reaction. Comparing to Si-MCM-41, the samples exhibited better conversion and higher selectivity for cumene cracking. The better catalytic performance is attributed to the samples possessing uniform channels, appropriate pore diameter, and active sites caused by the introduction of Ni and Co atoms into the framework of molecular sieves. Figure 7. 7. Diagram forcumene cumene cracking. Figure Diagramof ofthe the apparatus apparatus for cracking. Figure 7. Diagram of the apparatus for cumene cracking. Figure 8. Chromatogram (a,a*)and andconversion conversion (b,b*) cumene cracking reaction. Figure 8. Chromatogram (a,a*) (b,b*)curves curvesofofthe the cumene cracking reaction. 3. Conclusion Mesoporous heteroatom molecular sieves MCM-41s with crystalline phase were synthesized via a one-step hydrothermal method. The resulting materials possessed a structure in the vertical cross direction with a crystalline size of about 12 µm and high specific surface areas, and possess a highlyordered mesoporous structure and ordered arrangement channels. The elements in the samples have a uniform distribution and are highly dispersed in the materials. A certain amount of metal complexes formed by metal ion and EDTA is an essential factor for the formation of the vertical cross Crystals 2017, 7, 89 9 of 11 3. Conclusion Mesoporous heteroatom molecular sieves MCM-41s with crystalline phase were synthesized via a one-step hydrothermal method. The resulting materials possessed a structure in the vertical cross direction with a crystalline size of about 12 µm and high specific surface areas, and possess a highly-ordered mesoporous structure and ordered arrangement channels. The elements in the samples have a uniform distribution and are highly dispersed in the materials. A certain amount of metal complexes formed by metal ion and EDTA is an essential factor for the formation of the vertical cross structure. At the same time, the crystalline mesoporous heteroatom samples improve the alkali resistance of the molecular sieves. Compared to Si-MCM-41, the samples exhibited better conversion and higher selectivity for cumene cracking. 4. Materials and Methods Nickel nitrate hexahydrate, Ni(NO3 )2 ·6H2 O (AR, Beijing Yili Fine Chemical Product Limited Company, Beijing, China), was used as Ni source. Cobalt nitrate hexahydrate, Co(NO3 )2 ·6H2 O (AR, Shantou Xilong Chemical Limited Company, Shantou, China), was used as Co source. Cetyltrimethyl ammonium bromide (CTAB, AR, Tianjin Jinke Fine Chemical Institute, Tianjin, China) was used as structure template. Ethylenediaminetetraacetate (EDTA, AR, Beijing Chemical Factory, Beijing, China) was used as structure-directing agent. White carbon black (SiO2 , AR, Beijing Chemical Factory, Beijing, China) was used as Si source, and sodium hydroxide (NaOH, AR, Beijing Chemical Factory, Beijing, China) was used to adjust the pH. The new crystalline sample of Cry-M (M = Ni, Co) was prepared through a direct hydrothermal synthesis method, and the detailed process is as follows: a transparent solution of the ionic complex [M(EDTA)]2− was made by dissolving disodium ethylenediaminetetraacetate solution and a certain amount of M(NO3 )2 ·6H2 O in H2 O (10 g) and mixing well. CTAB (4.5 g, 12.3 mmol) was dissolved in 34 g of H2 O at 70 ◦ C. In another flask, 0.5 g (12.5 mmol) of sodium hydroxide dissolved in 15 g of H2 O was mixed with 2.25 g (37.5 mmol) of SiO2 stirred for 0.5 h and then heated to 60 ◦ C. The ionic complex [M(EDTA)]2− and CTAB solutions were then added to the silica solution, and the resulting mixture was stirred for 0.5 h to form a homogeneous emulsion. After adjusting the pH to 11 using sodium hydroxide solution, the mixture was stirred for another 12 h. The mixture was then transferred into a Teflon-lined autoclave and heated at 160 ◦ C under autogenous pressure for 96 h. The resulting solid was filtered and washed with water until the pH of the filtrate reached 7.0. After drying the sample overnight at 100 ◦ C, the template was removed by calcination in air at 550 ◦ C for 5 h. X-ray powder diffraction (XRD) patterns were recorded on a Philips X0 Pert diffractometer equipped with a rotating anode using Cu Kα radiation (λ = 0.1541 nm). The pore morphology of the samples was examined by high-resolution transmission electron microscopy (HRTEM) on a JEM-2100 microscope with an accelerating voltage of 200 kV. The crystal morphology was studied using a Hitachi S-4700 scanning electron microscope (SEM). The chemical compositions of the samples were measured by Optima 8000 inductively coupled plasma (ICP). Nitrogen adsorption–desorption isotherms were obtained using a Quantachrome Autosorb-1 volumetric adsorption analyzer. The volume of the adsorbed N2 was normalized to standard temperature and pressure. Prior to the measurements, the samples were degassed at 300 ◦ C for 16 h. The X-ray photoelectron spectroscopy (XPS) analyses were conducted on an ESCALAB 250 spectrometer equipped with an Al Kα X-ray source. The carbon 1 s peak at 284.6 eV was used as the reference for binding energies. X-ray absorption fine structure (XAFS) spectroscopy of the M (Ni, Co) K-edge of the samples was performed on the U7C beamline at the National Synchrotron Radiation Laboratory (NSRL) of China. The typical energy of the storage ring was 0.8 GeV, with a maximum current of 250 mA. A Si (111) double crystal monochromator was used. The data analysis of all the XAFS spectra was carried out using the IFEFFIT software (NSRL, Beijing, China). Crystals 2017, 7, 89 10 of 11 Acknowledgments: The work was supported by the National Natural Science of Foundation of China (No. 21271017) and the Fundamental Research Funds for the Central Universities (YS1406), Beijing Engineering Center for Hierarchical Catalysts. 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