Interest in multifunctional hybrid organic-inorganic materials is steadily growing due to their applications in several advanced technological fields [1, 2, 3]. Hybrid materials are of two distinct classes [4]: materials with the organic component physically embedded in an inorganic porous matrix(Class I) and hybrid phases with covalently bonded organic and inorganic elements integrated at the molecular level(Class II). The latter have more potential because they combine the high thermal，mechanical，and structural stability of the inorganic component with the easy functionalization and flexibility of the organic groups. Additional advantages exist when the hybrid material is crystalline and porous; the later provides shape selectivity by limiting molecular accessibility inside the particles [5, 6, 7, 8, 9]. As an example，one can cite the functionalization of zeolites with organic groups to modify framework hydrophobicity and introduce new functionalities useful for catalysis，optics，and sensing. Early attempts at grafting organosilanes on preformed zeolites [10, 11] were largely unsuccessful. The direct synthesis with a mixture of TEOS and organosilane(co-condensation route)was demonstrated but this has limited applicability [12, 13, 14, 15]. Hybrid materials have been prepared by pillaring two-dimensional(layered)zeolites [16] with bridged silsesquioxane precursors. Two examples have been reported: pillaring of a MWW-type precursor with 1，4-bis- (triethoxysilyl)-benzene(BTEB)[17] and a layered UTL-type phase with bridged silsesquioxane precursors of various complexity [18]. By this approach，it is possible to modulate the porosity by selecting the appropriate bridged silsesquioxane precursor and its concentration with respect to the inorganic phase. But the pillared materials are only partially ordered as nearly regular stacks of r and omly oriented zeolite laye rs [17, 18].
The replacement of the framework oxygen atoms with small organic bridging groups is also possible. Different research groups have reported the synthesis of hybrids with the MFI，LTA，Beta，FAU and MOR framework structures using bis-(triethoxysilyl)-methane or ethane as the only Si sources [19, 20, 21, 22, 23, 24]. This approach gave partial functionalization with carbon contents as high as 25% of framework oxygen atoms. However，the incorporation of the organic groups in the zeolite framework was doubtful. In fact，none of the analytical methods reported by the authors could show whether they were located inside the crystalline structure or the amorphous phase [19, 20, 21, 22, 23, 24]. A breakthrough was the discovery of the family of crystalline materials called Eni carbon silicates(ECSs)[25]. The crystal structure solutions for several ECSs definitively demonstrated the incorporation of the organic moiety in the inorganic framework [25, 26, 27, 29]. The inorganic layers are composed of [AlO4] tetrahedra，spacing apart the four different [CSiO3] units，to form low density structures with different pore systems [25, 26, 27, 29]. The inorganic layers have strong analogies with zeolites since they can be built using secondary building units(SBUs)typical in microporous frameworks(e.g. SBUs 4=1 and 4 in ECS-3 [26]，SBU 3 in ECS-17 [27]) and can contain large aluminosilicate rings(e.g. two independent 8-rings in ECS-3 [26]，10-rings in ECS-17 [27]).
Having recognized these similarities，we tried to modify the inorganic layers by the complete substitution of Al with B，as is frequently done for zeolites [28]. The syntheses were largely unsuccessful，and often produced partially ordered phases such as ECS-4 with BTEB [25]. In contrast，the use of equi-molar NaAlO2 and H3BO3(as B source)led to the crystallization of ECS-14 with BTEB [29]，ECS-5 with 4，4’-bis-(triethoxysilyl)- biphenyl(BTEBP) and ECS-13 with 2，6-bis-(triethoxysilyl)- napthalene(BTEN)[30]. The crystal structure of ECS-14 resembles that of AlPO-5，an aluminophosphate with the AFI framework-type，strengthening the analogy between ECSs and zeolites [29]. ECS-13 is similar to another aluminophosphate(AlPO-C with the APC framework-type)，but it has a different tetrahedra connectivity [30]. Despite the large H3BO3 concentration in the reaction mixture，only small amounts of B were present in the ECS phases，which preferentially incorporate Al as the trivalent element [30]. Nevertheless，the addition of boric acid to the reaction mixture has positive effects on the crystallization of these phases，which was much faster than in the Si/Al system(a few days instead of weeks)，and enhances the yield，purity and size of crystallites.
The catalytic test reactions of the ECSs were favorable. ECS-14 proved to be more active than basic Zn-Al and Mg-Al hydrotalcite catalysts in the Claisen-Schmidt condensation of benzaldehyde and acetophenone [29]. ECS-3 was found to be an excellent basic catalyst for the trans-esterification of triolein with methanol to biodiesel [31].
The promising synthesis and catalytic results prompted us to change the composition of the inorganic layers by replacing Al with other trivalent elements. Ga was selected since its incorporation in the framework of several zeolites is well known [32, 33]. Furthermore，Ga containing zeolites possess interesting catalytic properties. In particular，galliumsilicate zeolites with the MFI topology are used in the “Cyclar” process where C3-C5 alkanes are transformed into aromatics [34]. Therefore，the possibility of obtaining zeolite-like hybrid materials with functionalization of both the organic and inorganic parts would allow their properties to be varied further for specific catalytic applications. In this paper we report the exploratory syntheses of ECS materials prepared using Ga instead of Al，and using BTEB as the bridged silsesquioxane precursor. In particular，the synthesis condition allowed:(1)isomorphous substitution(complete replacement)of Al by Ga in the crystalline structure of two previously reported ECS materials(ECS-3 and ECS-17) and (2)direct synthesis of a Ga-ECS-17 contracted phase，previously obtained only by the dehydration of the parent ECS-17. We shall refer to this Ga containing phase as Ga-ECS-17C，where “C” refers to the “contracted” form. Ga-ECS-17C and ECS-17 when dehydrated have the same strongly reduced unit cell volumes compared to ECS-17.
1，4-bis-(triethoxysilyl)-benzene(BTEB，JSI Silicone，98% purity)，gallium tri-isopropoxide(Alfa Aeser)，boric acid(Fluka，99.5 purity)，sodium aluminate(Carlo Erba Reagents，54 wt% Al2O3)，NaOH and KOH(both from Aldrich，99% purity)were used as purchased.
Ga-ECS materials were synthesized using BTEB as the sole silica source，gallium isopropoxide，boric acid and NaOH or KOH as the metal hydroxide(MeOH). The mixtures were prepared by adding the required amount of BTEB to a solution obtained by dissolving MeOH and Ga(OCH(CH3)2)3 in demineralized water. When required，H3BO3 was added after the dissolution of the alkaline source [30]. The resulting mixture was charged into a stainless steel oscillating autoclave and heated under autogeneous pressure at 100 °C for the desired time. After cooling to room temperature，the solid product was separated from the mother liquor，washed with demineralized water and dried overnight at 80 °C. The selected synthesis conditions that gave the best crystallization of the Ga-ECS materials and the results of the qualitative X-ray diffraction(XRD)characterization are reported in Table 1. Al-containing ECS-3 and ECS-17，runs 5 and 6，were prepared previously [26, 27] using sodium aluminate instead of gallium isopropoxide.
XRD data was collected using a PANalytical EMPYREAN diffractometer equipped with a RTMS(real-time multiple strip)PIXcel 3D detector in the Debye-Scherrer geometry. Data were collected in the continuous mode over the 3° ≤ 2θ ≤ 110° angular region with a scan speed of 0.0095°/s. Cu Kα radiation(λ = 1.54178 Å)was used. The atomic coordinates obtained from the refined structure of dehydrated ECS-17 [27] were used as a starting model for the crystal structure refinement of as-synthesized Ga-ECS-17C，which was carried out using the General Structure Analysis System(GSAS)package [36, 37]. Geometric soft constraints were applied to the tetrahedra [Si−O(1.61(2)Å)，Ga−O(1.83(2)Å)，O−O(2.60(1)，and 3.00(1)Å for the tetrahedral central atom of Si or Ga]. The constraint weighting factor was gradually decreased during the refinement to yield reasonable bond lengths. Atoms with the same structural role were constrained to have the same isotropic thermal displacement parameters，which were fixed during the refinement. A rigid body unit was used to model the aromatic ring allocation during refinement.
An Agilent V-500 NMR instrument was used to observe the 13C(126 MHz) and 71Ga(152 MHz)signals of the sample contained in 4 mm rotors spinning at 14 kHz. 71Ga NMR spectra were collected using samples(ca. 30 mg)hydrated for 7 d at 52% relative humidity(25 °C，over aq. NaHSO4)，with air-tight rotors(for liquids)to prevent dehydration under MAS，a Hahn spin-echo pulse sequence with an echo time of one rotor period(71.4 μs)，left-shifting(4 points)to the top of the echo，1s delay and shifts referenced to aq. Ga(NO3)3(0 ppm). 13C spectra were collected using a composite(90°-180°-180°)excitation pulse，“onepuldpth” with 16-phase cycling to suppress the Kel-F spacer signal，which was adapted from [38]，3.8 μs = 90°，spinal decoupling，30 s relaxation delay and shifts referenced to adamantane(38.5 and 29.4 ppm). A Bruker ASX-300 was used to observed 29Si(59 MHz)signals using samples(ca. 300 mg)contained in air-tight 7 mm rotors spinning at 5 kHz，3.8 μs(60°)pulse，90 s delay，mlev16 1H decoupling and shifts referenced to solid tetrakis(trimethylsilyl)silane(-9.8 and -135.2 ppm).
Morphological observations were performed with a field emission scanning electron microscope(SEM)JEOL JSM-7600F operated at an accelerating voltage of 2 kV in gentle-beam mode. The images were collected from powders dispersed in isopropanol and deposed. To prevent charging，the samples were coated with a thin layer of Pt. Energy dispersive spectrometry(EDS)analysis was conducted to obtain the Si/Ga atomic ratio in Ga-ECS. The data were collected at 15 kV on samples embedded into epoxy resin，polished and made conductive by the deposition of a thin layer of carbon. The data were collected by an Oxford INCA EDS system(detector X-Max 20)on the SEM JEOL JSM-7600F.
Thermal analysis(TG-DTA-MS)was performed with a Seiko TG/DTA 6300 thermobalance equipped with an alumina furnace. Data were collected from 30 to 950 °C，with a heating rate of 10 °C/min with a steady flow of air(50 ml/min).
As shown in Table 1，the synthesis conditions that drove the crystallization of the Ga containing ECS were very similar to those used for the Al-containing ones. In particular，ECS-3 was obtained at the molar ratio Si/Al(or Ga)= 1.2，and ECS-17 was obtained at the molar ratio Si/Al(or Ga)= 2，3.
The main differences in the Al and Ga synthesis conditions regarded the type of MeOH: ECS-3 and ECS-17 required the presence of KOH in addition to sodium from NaAlO2. In contrast，Ga-ECS-3 and Ga-ECS-17 were obtained with NaOH or a mixture NaOH/KOH(Run 2). Crystallization of Ga-ECS-17 with only NaOH as MeOH(Run 3)was favored by the addition of boric acid. The MeOH/Si ratios were very different between the Ga and Al synthesis，in particular for ECS-3. However，the total amount of alkaline metals present，considering also sodium derived from NaAlO2，were similar(Table 1). So，a(Na+K)/Si ratio near unity appears necessary for Ga-ECS synthesis. When only potassium was present，Ga-ECS-17C was obtained.
The material from Run 1 in Table 1 shows an XRD pattern very similar to that of ECS-3(Fig. 1). No other phases were seen in XRD analysis. The EDS analysis found a Si/Ga ratio equal to 1.3，close to the Si/Al ratio of ECS-3. No amorphous content was seen by SEM(Fig. 2). This new material is an agglomeration of nanocrystals that resembles the grain of rice morphology of ECS-3. The ECS phase，as described by Bellussi et al. [26]，is the first crystalline aluminosilicate hybrid with an open microporosity produced by a regular arrangement of inorganic and organic layers. The inorganic layer has several features typical of zeolites. It is built with zeolitic 4=1 secondary building units(SBU)interconnected by additional four-membered rings(4MR). Furthermore，there are two crystallographically independent 8MR with free diameters of 5.5 ⨯ 2.3 and 4.8 ⨯ 2.7 Å，respectively. That makes it particular interesting for catalysis，as proposed by Macario et al. [31]. NMR analysis confirmed that the gallium site is four-coordinate with only one 71Ga signal near 200 ppm(Fig. 3c).
The larger ionic radius of the Ga3+ cation compared to Al3+，and the smaller T-O-T bond angle in the Si-O-Ga system [35]，compensated each other to leave the cell parameters almost unchanged. This was verified by the positions of the diffraction peaks(Fig. 1)，which were indexed on a monoclinic unit cell with parameters very close to those for ECS-3 [26]. The main differences in intensities were attributed to the different scattering factors for Ga and Al.
29Si MAS NMR revealed that every silicon atom was covalently bonded to 3 oxygens and 1 carbon to give “T sites” with shifts -55 to -70 ppm(Fig. 3a top). In general，substituting Ga for Al caused a 4 ppm downfield shift(-65 to -61 ppm，Table 2). This corresponds to a 2.4° decrease in the T-O-T angle using the classical relation known for aluminosilicates [39]. 13C MAS NMR showed that the phenylene group was intact and bonded to two silicon atoms(Fig. 3b top). One-sided cleavage of the phenylene would give a signal at 129 ppm [40]. All these observations agreed with the synthesis of Ga-ECS-3.
The XRD pattern of the solid from Run 2 in Table 1 is very similar to that reported for as-synthesized ECS-17(Fig. 4)[27]. In this case，traces of gallium-sodalite were detected by XRD. Sodalite association is common because it is difficult to reach the equilibrium condition favoring hydrolysis of the bridged silsesquioxane precursor without causing Si-C bond rupture which can，under basic hydrothermal conditions，lead to the crystallization of a conventional zeolite. The Si/Ga molar ratio is exactly the same as the Si/Al molar ratio in ECS-17 [27]. ECS-17 and the new gallium hybrid have similar morphologies(Fig. 5)although a higher tendency for intergrowth was noticed for the latter. ECS-17 is the first crystalline hybrid organic-inorganic aluminosilicate built from only 3-ring SBU [27]. Its framework was defined reversible “collapsible” following the Baur classification of zeolite thermal behavior [41].
The hybrid nature of Ga-ECS-17 was confirmed by 29Si NMR spectroscopy，which showed mainly T sites(Fig. 6a top). A small amount of Q sites(SiO4)，related to the gallium-sodalite impurity，was also present(signals near -85 ppm). Only a GaO4 site was detected(Fig. 6c) and the phenylene group remained intact(Fig. 6b top).
The extra-framework cation content of ECSs depends on the type of mineralizing agent(usually NaOH or KOH) and the aluminum source(i.e. NaAlO2). Substituting sodium aluminate with gallium isopropoxide as the trivalent atom source is a way to modify the extra-framework cation. The first trial was made in Run 4(Table 1)，where K+ was the only cation balancing the negative framework charge(GaO4-). The result was a purely crystalline material with a diffraction pattern resembling that of ECS-17 dehydrated at 120 °C under vacuum [27] but with a higher degree of crystallinity(Fig. 7). The structure was more compact with a remarkable volume decrease(about 17%)，compared to the as-synthesized phase，due to the loss of water molecules around the cations. This new Ga-containing phase does not have “zeolitic” water. TG analysis showed a slight weight loss(1.8%)near 100 °C related to physically adsorbed water(Fig. 8). The successive strong weight loss above 350 °C resulted from the decomposition of the phenylene groups. In its morphology，the new Ga-ECS-17C(with K+)，composed of regular prismodic crystals with a high tendency for intergrowth(Fig. 9)，is similar to ECS-17 and Ga-ECS-17. NMR experiments confirmed the strong similarities between the dehydrated form of ECS-17 and Ga-ECS-17C(Fig. 10)[27]. More importantly，dehydrated ECS-17 has the same structure as Ga-ECS-17C obtained from direct synthesis with potassium. This can be readily explained by the larger ionic radius of K+， occupying the center of the 10-MR，compared to Na+. Steric hindrance prevents the access of water molecules trying to solvate the extra-framework species in the channels. As a result，potassium exercises a “dragging effect”. This effect，known to zeolite crystallographers [42]，provides more efficient coordination of framework oxygen atoms causing the 10-membered rings to shrink. It was observed for the ECS-17 de hydrated phase. This configuration was confirmed by structural refinement，which converged to satisfactory discrepancy factors，starting from the structural model for dehydrated ECS-17 [27](Fig. 11)where the coordination sphere of K+ contains six framework oxygens.
29Si MAS NMR spectroscopy confirmed the high purity of this product，without any Q site(SiO4)signals(Fig. 10a top). 13C chemical shifts showed very little difference between the chemical environment of the phenylene group in Ga-ECS-17 and Ga-ECS-17C(Table 2).
Ga-ECS-3 and Ga-ECS-17 are the first synthesized crystalline organic-inorganic hybrids containing Ga. They have the same crystal structures as their aluminum forms，ECS-3 and ECS-17. The contracted(dehydrated)ECS-17 topology was obtained by direct synthesis(using K+)without “zeolitic” water. This offers new crystal chemistry for ECS materials，which can now be functionalized by organic [25] and inorganic components. Access to a new metal source eliminates sodium as an extra-framework component. Indeed，similarly to zeotype materials，ECSs are able to(1)accept isomorphous substitution of heteroatoms by adjusting their energetic，and therefore，structural configuration，and (2)hold different extra-framework cations and adapt the negative charged framework to a more favorable coordination.
2015, Vol. 36 Issue (6): 813-819
PDF (734 KB)
2015, Vol. 36 Issue (6): 813-819
PDF (734 KB)