Aluminated Derivatives of Porous Magadiite Heterostructures for Acid

Article pubs.acs.org/IECR

Aluminated Derivatives of Porous Magadiite Heterostructures for Acid-Catalyzed tert-Butylation of Catechol Qi Sun, Cong Zhang, Huan Sun, and Hui Zhang* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing 100029, China S Supporting Information *

ABSTRACT: Novel porous magadiite/Al-magadiite heterostructures (PMH/PAMH) and aluminated derivatives of PMH (xAlPMH, x = Al/Si in feeding) were fabricated upon two-dimensional interlayer cosurfactant-directing TEOS hydrolysis− condensation−polymerization from synthetic Na-magadiite/Na-[Al]magadiite and postgrafting of Al into the interlayer silica framework of PMH from NaAlO2 precursor, respectively. Characterization studies indicate that PMH and PAMH possess high surface area (SA), high thermal stability, and unique supermicro−mesoporous structure upon effective assembly of interlayer mesostructured silica and clay layers but weak Lewis acidity. The xAl-PMH (x = 0.2, 0.4) samples show successful incorporation of Al into interlayer mesostructure of PMH mainly in tetra-coordinated form, leading to greatly increased Lewis acidity and newly created Brønsted acidity together with well-kept layered supermicro-mesoporous porosity and reduced SA (>280 m2/g) while 0.6Al-PMH shows collapsed layers. 0.4Al-PMH exhibits the highest liquid-phase Friedel−Crafts tert-butylation activity of catechol with 93.4% conversion and 80.4% 4-tert-butylcatechol selectivity due to the strongest synergy between the surface acidity and supermicro−mesostructure.

1. INTRODUCTION Porous solid materials are of great importance in catalysis because of their ability to interact with atoms, ions, and molecules not only at their surfaces but throughout the bulk of the material.1 In 1970s, pillared interlayered clays (PILCs) have been extensively studied as acidic alternatives to zeolites for better processing of crude oil to deal with the oil crisis.2 The pillared clays including bentonite, montmorillonite, and saponite upon the intercalation of polymers or cationic compounds of Al,3 Fe,4 Si,5 and Co6 into the interlayer space of clays produce PILCs materials with relatively large surface area and controllable pore widths ranging from 0.4 to 1.8 nm, implying larger pores than those of zeolites in favor of the diffusion of bulky reactant molecules.2 However, the pillars’ structure of these PILCs materials easily collapse at relatively mild temperatures (400 °C), and their acidities are lower, which greatly limit the catalytic applications of the PILCs composites. Fortunately, the successful synthesis of well-ordered mesoporous silica via liquid crystal template in 1990s7 caused an idea, originally proposed by Pinnavaia,8 of applying the approach for preparing mesoporous silica to clay-based new materialsporous clay heterostructures (PCHs). The PCHs materials are characterized by high surface area, good thermal stability, microporous and mesoporous structure, and surface acidity.9 All these special features offer PCHs new opportunities for selective heterogeneous catalysts.9,10 Though the acidity of PCHs could be increased upon acid-activated host clay,11 varied strategies have been developed to enhance the acidic functionality by intercalating silica−aluminia12 and silica− titania13 pillars or by postsynthesis Al grafting14−16 similar to what’s developed for the modification of MCM-41,17 given that none or small amount of acid sites located in the interlayer © 2014 American Chemical Society

silica framework apart from those aroused from the parent clays. Up until now, various types of cationic clays such as fluorohectorite,8 montmorillonite,12−14,18 synthetic saponite,14−16 and vermiculite18 were used as precursors for preparing PCHs, while pure siliceous magadiite was seldom studied probably due to its difficult swelling originated from higher layer charge density.8,10 Magadiite, a kind of hydrous sodium silicate first discovered in lake beds at Lake Magadi,19 can be readily prepared in laboratory by hydrothermal method.20,21 Upon synthesized pure siliceous magadiite, a better candidate for PCH materials can be expected with excellent resistance to acid, hydrothermal stability, controllable metal-doping, and desired acidity compared with common clays. Though Pinnavaia et al.8 mentioned magadiite in their first report on PCHs from varied clay hosts, no detailed characterization and catalysis studies have been published so far, far from its designed acidic modification upon varied routes by incorporating Al in host layer or grafting Al into interlayered silica of PCHs for desired acid-catalyzed process. Kwon et al.22,23 has ever reported an improved synthesis step of mesoporous silica-pillared H+magadiite by amine-assisted TEOS hydrolysis without preswelling; however, the obtained pure silicic materials yet hold very low or no acidity. Herein, we report novel porous magadiite/Al-magadiite heterostructures (PMH/PAMH) and aluminated derivatives of PMH (xAl-PMH, x = Al/Si) upon two-dimensional interlayer cosurfactant-directing TEOS hydrolysis and conReceived: Revised: Accepted: Published: 12224

June 5, 2014 July 10, 2014 July 14, 2014 July 14, 2014 dx.doi.org/10.1021/ie502266r | Ind. Eng. Chem. Res. 2014, 53, 12224−12237

Industrial & Engineering Chemistry Research

Article

Scheme 1. Representation of Porous Magadiite/Al-Magadiite Heterostructures (PMH/PAMH) via 2-Dimensional Interlayer Cosurfactant-Directed TEOS Hydrolysis and Condensation-Polymerization Assembly and Aluminated Derivatives of PMH (xAl-PMH) by Postgrafting Al Using NaAlO2

transferred into a Teflon lined stainless steel autoclave for hydrothermal treatment under autogenous pressure at 150 °C for 48 h. The resultant was filtered and washed with deionized water and air-dried at 60 °C giving the Na-magadiite with an empirical formula of Na1.90Si14.00O29.34·7.86H2O (see Supporting Information Table S1), very close to previously reported Na 1.96 Si 14.00 O 28.89 ·8.00H 2 O by Park et al., 23 based on thermogravimetric (TG) (Supporting Information Figure S1a) and energy dispersive X-ray (EDX) data (inset in Supporting Information Figure S4). The Na-[Al]magadiite was prepared from a mixture composed of SiO2/Al2O3/NaOH/ H2O (9/0.45/2/75) under hydrothermal treatment at 150 °C for 72 h. Then, 5.0 g Na-magadiite/Na-[Al]magadiite (∼4.8 mmol) was mixed with 100 mL aq. solution of CTMABr (0.137 M) under vigorously stirring at 80 °C for 4 h, followed filtering and washing with deionized water and drying at 60 °C overnight to form CTMA+-magadiite/Al-magadiite (QM/QAM in short). The empirical formula for QM/QAM (Supporting Information Table S1) was estimated upon the empirical formula of magadiite, assuming that the amount of Na+ ions exchanged for CTMA+ corresponds to the cation exchange capacity of clay. A mixture of 2.0 g hydrated QM/QAM (∼1.4 mmol), 4.4 g n-decylamine (28.0 mmol), and 43.7 g TEOS (209.8 mmol) were allowed to react. After 8 h interaction at room temperature, during that period a hydrous silica templated around a monolayer of micellar CTMA+ and ndecylamine assemblies is formed,8 the solid was centrifuged and dried in air at 60 °C overnight to further promote intragallery TEOS hydrolysis leading to a mediate product PQMH/ PQAMH. The PQMH/PQAMH was calcined at 550 °C for 4 h giving porous magadiite/Al-magadiite heterostructures (PMH/PAMH). Then, the Al-grafted PMH samples were prepared by postsynthesis route (Scheme 1 steps IV−VI). Carefully weighed 0.5 g PMH was well dispersed in 50 mL NaAlO2 solutions with varied concentrations (0.032/0.064/ 0.096 M) according to designed Al/Si molar ratio (0.2/0.4/0.6) under vigorously stirring at room temperature. The resultants

densation-polymerization from synthetic Na-magadiite/Na[Al]magadiite and postgrafting of Al into the interlayer silica framework of PMH from NaAlO2 precursor, respectively. The obtained samples were systematically studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Brunauer−Emmett−Teller (BET), 29 Si/27Al MAS NMR, NH3-TPD, and pyridine Fourier transform infrared (FT-IR) methods. The influence of both Al location (layer or interlayer) and contents on the acidity of the samples is thoroughly discussed. The xAl-PMH samples present dramatically high catalytic efficiency for the liquidphase Friedel−Crafts tert-butylation of catechol to paraselectively produce 4-tert-butylcatechol compared to PMH and PAMH due to the significantly enhanced Lewis acidity and newly created Brønsted acidity and well-kept layered supermicro−mesoporous structure.

2. EXPERIMENTAL SECTION 2.1. Materials. Silicate gel, n-decylamine, and alumina gel were obtained from Sigma-Aldrich, cetyltrimethylammonium bromide (CTMABr) from Tianjin Jinke Fine Chemical Factory (China), tetraethylorthosilicate (TEOS) from Xilong Chemical Limited Corporation (Guangzhou, China), NaAlO2 from Tianjin Guangfu Fine Chemical Factory (China), and catechol, tert-butylalchol, and p-xylene from Aladdin Reagent Limited Corporation (Shanghai, China). All chemicals are A.R. grade and used as received. 2.2. Synthesis of the Catalysts. Scheme 1 depicts the process for synthesizing PMH derived catalysts, in which steps I−III illustrate the synthesis of pure silicic PMH from magadiite and Al-doped PAMH from [Al]magadiite, and steps IV−VI depict the synthesis of xAl-PMH from PMH. First, Namagadiite was prepared via hydrothermal route using silicate gel and sodium hydroxide according to published methods.24−26 Typically, the starting mixture composed of SiO2/NaOH/H2O with molar ratio of 9/2/75 was stirred for 3 h and then 12225

dx.doi.org/10.1021/ie502266r | Ind. Eng. Chem. Res. 2014, 53, 12224−12237


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Figure 1. XRD (A) and FT-IR (B) spectra of PMH (a), PAMH (b), 0.2Al-PMH (c), 0.4Al-PMH (d), and 0.6Al-PMH (e).

was outgassed at 300 °C in pure N2 flow (20 mL/min) for 1 h. Then, the sample was naturally cooled to 100 °C, and NH3 was adsorbed by exposing sample treated in this manner to a stream of pure NH3 for 30 min. It was then flushed at 100 °C with N2 for another 30 min to remove physisorbed NH3. The desorption of NH3 was done in pure N2 flow (20 mL/min) by increasing the temperature to 800 °C with a liner heating rate of 10 °C/min and measuring NH3 desorption. Pyridine adsorption FT-IR spectra were recorded on a Nicolet 380 spectrometer. Transmission IR spectra (in absorbance mode) were recorded using self-supporting pellets of the sample powder. The pellet (diameter 13 mm) of 30 mg was placed in an IR cell designed to carry out spectroscopic measurement at varied temperatures and equipped with CaF2 windows. The samples were first heated to 400 °C at a heating rate of 10 °C/ min in a pure N2 flow, kept for 2 h, and then cooled to room temperature. The pyridine vapor was introduced under N2 flow for 1 h; subsequently, weakly adsorbed pyridine was flushed for 0.5 h under N2 flow. The FT-IR spectra were recorded at 150, 250, and 350 °C, and a resolution of 8 cm−1 is attained after averaging over 64 scans for all the FT-IR spectra reported here. 2.4. Catalytic Properties. The tert-butylation of catechol (CAT) with tert-butyl alcohol (TBA) was carried out in a batch glass reactor, equipped with reflux condenser and magnetic stirrer at atmospheric pressure. Reaction conditions: TBA/CAT molar ratio, 1.0−3.0; catalysts, 10 wt % of CAT; solvent, pxylene (9 mL); reaction temperature, 110−138 °C; reaction time, 1−8 h. A quantitative analysis of the products was conducted on a gas chromatograph (Agilent Technologies 7890A), equipped with an Agilent J&W HP-5 capillary column (5% phenyl polysiloxane, 30 m × 0.32 mm × 0.25 μm) and a flame ionization detector (flame ionization detection (FID), 340 °C; injector, 320 °C; temperature program, 120 °C (3 min) to 190 °C by 10 °C/min, kept for 0.5 min, and then to 270 °C by 40 °C/min and kept for 1 min). The conversion of CAT is defined as moles of consumed CAT to the moles of CAT employed. The selectivity to specific product is defined as moles of specific product divided by moles of consumed CAT. Authentic standard (Acros organics) and 1H NMR (Bruker AV600, 600 MHz) were used to identify the reaction products. The used catalyst was separated from reaction system by centrifugation, washed thoroughly with ethanol, dried at 120 °C followed calcination at 400 °C in air for removing residues, and subjected to the next run.

were centrifuged and washed thoroughly with deionized water. To get the protonated forms of Al-grafted PMH, the Na+ ions in the as-made aluminated products were exchanged for NH4+ by equilibrating in 40 mL NH4NO3 (1.0 M, 40 mmol) for 3 h, and then, the NH4+-exchanged derivatives were calcined at 600 °C for 4 h to form the protonated catalysts xAl-PMH (x is Al/ Si molar ratio in feeding). 2.3. Characterization. X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 0.1542 nm, 40 kV, 30 mA) in 2θ 2−60° with a scan speed of 10°/min by a step of 0.01°. Fourier transform infrared spectra (FT-IR) were obtained on a Bruker Vector 22 spectrophotometer in the range 4000−400 cm−1 with 2 cm−1 resolution and 60 scans by the standard KBr disk technique (sample/KBr = 1/100) with almost identical mass of the samples of ∼2 mg. SEM-EDX results were obtained on an Oxford Instruments INCAx-act EDX detector attached to a Zeiss Supra 55 field emission scanning electron microscopy using a 15 kV electron beam and 60 s acquisition time. Transmission electron microscope (TEM) and HRTEM images were obtained with Hitachi-800 and JEM-2010, respectively, operated at an accelerating voltage of 120 kV. Thermogravimetric analysis were taken on a Mettler Toledo TGA/DSC 1/1100 SF thermoanalyzer at a heating rate of 10 °C/min in 25−1000 °C under N2 flow (25 mL/min). Texture analysis was realized by low-temperature N2 sorption at 77 K on a Quantachrome AS-1C-VP automated gas adsorption system. The surface area was estimated using the BET method, micropore volume by using the t-method of DeBoer, and the pore size distribution by using both the Horvath−Kawazoe (H−K) and the Barrett−Joyner−Halenda (BJH) models. 27Al magic-angle spinning (MAS) NMR spectra of the powdered samples were obtained on a Bruker MH 400 Avance III spectrometer (9.4 T) at a resonance frequency of 104.2 MHz. The samples were placed in a 4 mm zirconia rotor with a spinning frequency of 10 kHz. A pulse duration of 0.25 μs and a pulse delay of 2 s were used. External aluminum nitrate solution (1.0 M) was used as reference to determine the chemical shift value. 29Si MAS NMR spectra were obtained at 79.5 MHz using a 7 mm zirconia rotor and a spinning frequency of 5 kHz. A pulse duration of 4.00 μs and a pulse delay of 60 s were used. The 29Si chemical shift is reported with respect to Kaolin standard (δ = −91 ppm). Temperature-programmed desorption of NH3 (NH3-TPD) was performed on a Thermo Electron Corporation TPDRO 1100 series instrument. Prior to NH3 sorption, 0.1 g sample 12226



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adsorbed water on the surface of Na-magadiite.27 The intense band at 1083 cm−1 is ascribed to Si−O−Si asymmetric stretchings, whereas two at 822 and 781 cm−1 appear overlapped in an intense band to Si−O−Si symmetric stretchings predominantly involving silicon motions (named as νs(Si−O−Si)), and two at 620 and 546 cm−1 to the bending modes of single and double Si−O−Si rings.27,29 For QM, two strong bands at 2919 and 2850 cm−1 correspond to antisymmetric and symmetric stretching ν(C−H) in CH2 groups, respectively, and a sharp one at 1487 cm−1 to CH3 antisymmetric and CH2 asymmetric scissoring modes, indicating the successful intercalation of CTMA+. For PQMH, strong IR bands for organic groups are observed due to the intercalation of cotemplates and TEOS, while the bending mode of water at 1628 cm−1 becomes quite weak, implying nearly complete hydrolysis and condensation−polymerization occurred. While for Na-[Al]magadiite series, Supporting Information Figure S3B shows similar FT-IR bands except the weakened and broadened νas(Si−O−Si) and νs(Si−O−Si), implying less ordered layered framework due to the Al incorporation. Then, for PMH and PAMH (Figure 1B(a,b)), all the bands related to the organic groups vanish implying the elimination of organic templates. A broad weak band at 3470 cm−1 can be assigned to hydrogen-bonding of the terminal silanol groups of PMH,29 which red-shifted to 3430 cm−1 and showed lower intensity for PAMH due to the layer Al-incorporating. Regarding the Si−O−Si stretchings, two sharp close bands at 822 and 781 cm−1 in the magadiite are replaced by a small single one at ∼807 cm−1, and those in 650−500 cm−1 completely disappear, while the peak at ∼461 cm−1 still remained. Note that the intense band at 1083 cm−1 and a welldefined shoulder at 1237 cm−1 can be ascribed to νas(Si−O−Si) and Si−O−Si stretchings of five-membered rings in calcined materials,25,29 respectively, suggesting considerable structure stability of magadiite sheets in both PMH and PAMH materials. Interestingly, the FT-IR of xAl-PMH (Figure 1B(c−e)) shows that the broad and weak Si−OH signals at 3470 cm−1 (PMH) not only shift to 3430 cm−1 but also reduce in intensities (lower than PAMH) due to the Al-grafting, indicating that the incorporation of Al into the interlayer space of PMH may cause reduced amount of Si−OH groups on the formation of Si − O − Al linkage. The less pronounced Si−OH bond, the higher Al content in the samples, indicating that 0.4Al-PMH holds the highest Al content. Moreover, the intensity of the band at 1083 cm−1 and its fair-defined shoulder (1237 cm−1) due to the Si− O−Si framework are greatly reduced with increasing Al-grafting for 0.2Al-PMH and 0.4Al-PMH, implying the increased amount of Al-grafting into the interlayer silica framework.30 The intensities of the bands at 807 cm−1 due to νs(Si−O−Si) exhibit similar changing trends. To the contrary, the higher intensity of the band at 1083 cm−1 for 0.6Al-PMH and PAMH may be due to the varied Al-coordinated forms as later 27Al MAS NMR revealed. The lowest intensity of the band at 461 cm−1 for PAMH implies the incorporation of Al into the layer of the parent clay. These results indicate the formation of new Si−O−Si and Si−O−Al linkages in xAl-PMH samples leading to corresponding symmetric and asymmetric stretchings.12,31 3.2. Morphology. The SEM/EDS and (HR)TEM images of PMH and PAMH are shown in Figure 2, and the morphological evolution of the intermediates shown in Supporting Information Figure S4. Clearly, the starting Namagadiite shows rosette morphology of ca. 15 μm (inset in

3. RESULT AND DISCUSSION 3.1. X-ray Diffraction and FT-IR Analysis. The XRD patterns of PMH, PAMH, and xAl-PMH samples are shown in Figure 1A. The XRD patterns of the starting magadiite and intermediates (Supporting Information Figure S2A) show that Na-magadiite displays a very strong peak at 2θ 5.9° (1.48 nm), and two weak ones at 11.7° and 17.4°, corresponding to (001), (002), and (003) lines (JCPDS 42-1350), indicating the formation of well-crystallized Na-magadiite.19,20,26 After ionexchanging, the QM shows an increased d001 of 2.93 nm due to the intercalation of CTMA+, similar to the d001 of 3.02 nm previously reported.25 Further intercalation of cosurfactant and TEOS hydrolysis and condensation-polymerization lead to the PQMH with a greatly expanded d001 of 3.06 nm. Then, the product PMH (Figure 1A(a)) exhibits a further expanded d001 of 3.32 nm, together with reduced intensity and broadened peak shape, which is probably attributed to the stabilization role of the cotemplates though with somewhat disturbance of layer ordering due to the removal of intragallery cosurfactants during the calcinations for the completion of the dehydroxylation and cross-linking of the gallery-assembled silica structure.8,13 These results clearly indicate the successful synthesis of novel pure siliceous PMH material. The XRD of Na-[Al]magadiite (Supporting Information Figure S3A) shows well-defined (001) peak with d001 of 1.46 nm, close to the literature value,26,27 implying the well-kept crystallinity of magadiite after introducing small amount of Al (nominal Si/Al = 10) into layered silica framework. Very similar evolutions to PMH occurred during the same CTMABr swelling and cotemplate and TEOS intercalating procedures. After calcination, the PAMH (Figure 1A(b)) depicts a similar basal spacing of 3.20 nm to PMH, indicating the successful synthesis of mesostructured PAMH material. Upon postgrafting, the xAl-PMH samples exhibit no additional XRD peak in the detected 2θ region (Figure 1A(c−e)), compared with pure PMH, indicating the absence of a segregated aluminum phase. The 0.2Al-PMH shows clearly sharp (001) peak (inset in Figure 1A) with d001 of 2.98 nm smaller than the pristine PMH probably due to the slight gallery contract in alkaline NaAlO2 solutions.16 With increasing Al-grafting, the (001) peak for 0.4Al-PMH is weaker but still distinguishable with d001 of 2.86 nm, while for 0.6Al-PMH it disappears in the background, implying the loss of long-range structural order. The incorporation of Al (Al−O bond length of 1.75 Å) into silica framework may be promoted by the cleavage of some Si−O bond (1.61 Å) in basic NaAlO2 solution,16 and Al3+ cations could therefore partially replace Si4+ in SiO4 tetrahedra of the interlayer silica framework. However, the long-range ordered structure of PMH was greatly damaged in an attempt to prepare samples with higher Al/Si ratio by adding more Al reagent in grafting process, as confirmed by the reduced (001) peak intensity, similar to Al-grafted MCM-41.28 The reducing d001 may also be ascribed to the destruction of interlayer integrity due to the Al-grafting into the intragallery of PMH. The FT-IR spectra of PMH, PAMH, and xAl-PMH are shown in Figure 1B, and the FT-IR spectra of the starting materials and intermediates shown in Supporting Information Figure S2B. The Na-magadiite exhibits an identical IR spectrum to those previously reported25,27 with a broad band in 3800− 3000 cm−1 together with narrow ones at 1672 and 1628 cm−1 assigned to the stretching and bending modes, respectively, of 12227



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serious dealumination during calcinations (discuss later in 27Al NMR). Note that the layers are still apparent without severe destruction, implying general maintenance of Al-magadiite layers on the formation of PAMH. The SEM and TEM of the postsynthesis xAl-PMH (Figure 3) show that all three samples exhibit a rugged and irregular

Figure 2. SEM, TEM, and HRTEM images of PMH (a, b, c) and PAMH (d, e, f).

Supporting Information Figure S4a) constructed by contact packing of well-defined plates with thickness of ∼50 nm and length of ∼2 μm, quite similar to previously reported.22,27 The QM shows loose discrete rectangle plate-like particles with very inhomogeneous size originated from broken rosette morphology of Na-magadiite due to the intercalation of CTMA+. The PQMH exhibits considerably swelling plate-like morphology, though these plates are quite irregular and some of them show an edge-curved shape due to the TEOS hydrolysis− condensation−polymerization within the magadiite interlayer. The morphological similarity between QM and PQMH implies that the intercalation of cosurfactants and TEOS occurs in a topotactic fashion. Then, the PMH (Figure 2a−c) shows typical well-crystallized layers (∼2 μm × 65 nm) with a smooth surface and regular packing probably due to the decomposition of interlayer organic groups and proper layer rearrangement during the calcinations. HRTEM images further reveal the progressively transformation of the parent clay to PMH. Intragallery space appears in light in contrast to the dark clay layers. The mean thickness of the dark layer is ∼1.29 nm (Figure 2c), similar to ∼1.17 nm of Na-magadiite layer (Supporting Information Figure S4g) and the literature value 1.12 nm,32 strongly suggesting the transformation of PQMH into PMH with no change of original layer structure of starting magadiite. The (001) lattice fringes of the magadiite, QM, PQMH, and PMH are 1.54, 2.72, 2.95, and 3.20 nm, respectively, in accord well with their XRD data. The EDS of PMH (inset in Figure 2a) reveals the existence of Si and O with Si/O molar ratio of 0.43, which is similar to the pristine magadiite (0.48), implying quite homogeneous intercalation of meso-silica into the interlayer space of magadiite. For PAMH series, the starting Na-[Al]magadiite (Supporting Information Figure S5a) shows similar morphology to Namagadiite. However, a closer look at one of these aggregates indicates that the plates are very inhomogeneous in size and some of them present a curved shape and the presence of Al (EDS in Supporting Information Figure S5a inset confirmed) has made the plates thicker in the borders, close to previously reported 30[Al]-magadiite by Superti et al.27 The morphological change for QAM and PQAMH follows the same trend as QM and PQMH. Then, the PAMH (Figure 2d−f) shows well-crystallized layers with regular packing but rather rough surfaces. HRTEM shows interplanar spacing of 3.15 nm, close to its XRD data. The layer thickness of PAMH varies in 1.35− 1.64 nm (Figure 2f), thicker than Al-magadiite layer (∼1.27 nm) probably due to the disturbance of the layer ordering upon

Figure 3. SEM (a, c, e) and TEM (b, d, f) images of 0.2Al-PMH (a, b), 0.4Al-PMH (c, d), 0.6Al-PMH (e, f) (insets refer to EDS and HRTEM of 0.4Al-PMH).

platy morphology compared to pure PMH. In detail, 0.2AlPMH (Figure 3a,b) and 0.4Al-PMH (Figure 3c,d) display similar particle size and layer morphology to PMH, though the TEM shows loosely packing plates, implying well-kept layer structures of xAl-PMH samples with relatively lower Al contents. Although the lamellar habit of each individual particle is well-preserved, a little curling by the edge and relatively roughness of platelets’ external surface can be clearly seen compared to PMH. These phenomena may be caused by the successful impregnation of heteroatoms into the silica framework. As for 0.6Al-PMH (Figure 2e,f), not only the particle size is reduced but the major rectangle platelets are evidently broken into nearly spherical aggregated particles also and the layered structure is severely collapsed, indicating a serious damage to the lamellar habit of pristine PMH. The EDX data show the bulk/intragallery Al/Si ratios of 0.07/0.14, 0.09/0.20 and 0.07/- for 0.2Al-PMH, 0.4Al-PMH and 0.6Al-PMH (Supporting Information Table S1), respectively, all lower than the feeding intragallery Al/Si ratio, but much higher than the bulk Al/Si ratio of PAMH (0.03), implying the difficult but better Al-incorporation with considerable Al content is obtained by postsynthesis route than from Na-[Al]magadiite. However, among all xAl-PMH samples, 0.6Al-PMH shows much lower Al content than the feeding one probably due to the strong alkaline solution from high NaAlO2 concentration 12228



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Figure 4. Low-temperature N2 adsorption−desorption isotherms (A) and H−K and BJH pore size distributions (B) of PMH (a), PAMH (b), 0.2AlPMH (c), 0.4Al-PMH (d), and 0.6Al-PMH (e).

Table 1. Physico-chemical Properties of the Catalysts with Different Al/Si Molar Ratios rpore (nm)d samples

GH (nm)

PMH PAMH 0.2Al-PMH 0.4Al-PMH 0.6Al-PMH

2.20 2.08 1.86 1.74 −

a

2

b

2

SBET (m /g)

SMicrop (m /g)

729.2 741.2 369.6 282.5 187.3

176.3 193.0 138.9 45.2 94.7

c

2

S (m /g) 552.5 544.8 230.4 237.3 92.6

c

VMicrop(cc/g)

VTotal(cc/g)

micro

meso

0.090 0.092 0.068 0.024 0.049

0.66 0.56 0.35 0.27 0.24

0.80 0.76 0.78 0.78 0.78

1.96 2.20 1.92 1.90 1.88

Gallery height (GH) = basal spacing − 1.12 nm (thickness of magadiite layer). bCalculated from N2 adsorption isotherm according to BET method. SMicro and S are microporous and nonmicroporous surface areas obtained from the De Boer t-plot method. dCalculated from the N2 desorption branch using the HK and BJH model, respectively.

a c

Then, the xAl-PMH samples (Figure 4A(c−e)) exhibit similar IV type isotherms with H4 type hysteresis loop. Differently, the quite lower accumulated adsorption amounts than PMH and PAMH imply possibly reduced surface area and the flatter but larger separations between desorption and adsorption branches than PMH and PAMH indicate a relatively narrower PSD. The hysteresis loops initiated at even much lower p/p0 especially for 0.2Al-PMH and 0.4Al-PMH imply their slightly broader micropore PSD besides the mesopore PSD. The slightly flatter hysteresis loop and a slightly steep linear uptake of N2 within p/p0 0.05−0.3 of 0.6Al-PMH imply its slightly pronounced micropore PSD features than 0.2AlPMH and 0.4Al-PMH. These phenomena apparently indicate more Al grafted into the intragallery silica framework in xAlPMH (x = 0.2, 0.4) than in 0.6Al-PMH. The BET surface area and De Boer t-plot data (Table 1), and PSD on both HK and BJH models (Figure 4B) rationally support above analysis. The optimum pore sizes include rmicro of 0.78 ± 0.02 nm and rmeso of 1.90 ± 0.02 nm, slightly smaller than those of PMH and PAMH, confirming the well-kept supermicropores and small mesopores in present catalysts. These PSD results are in agreement with the estimated gallery height upon XRD (Table 1). The SBET of PMH is 729.2 m2/g, close to that of PAMH (741.2 m2/g) attributed to their similar synthesis step and low Al content in PAMH. The sequentially reduced SBET for 0.2Al-PMH, 0.4Al-PMH, and 0.6Al-PMH indicate that the postgrafting of Al into PMH obviously causes a gradual decrease in surface area. Note that 0.4Al-PMH has the minimum Smicro and Vmicro, while 0.6Al-PMH possesses the largest proportion of microporous area, probably due to more Al grafted into the intragallery silica framework in the former while relatively larger amount of Al on the surface of PMH in the latter. Actually upon EDX of PMH and xAl-PMH (Supporting Information Table S1), the high intragallery Al/

leading to obvious dissolution of Al and simultaneous disturbance of the layered structure of PHM. These observations explain for the loss of structural regularity as XRD showed and further confirm that the Al/Si stoichiometry in ambient solution is an extremely important factor for successful Al-grafting with well-ordered layer structure. A typical HRTEM of 0.4Al-PMH (inset in Figure 3d) clearly shows an interplanar spacing of 2.95 nm close to its XRD data. The mean layer thickness of 0.4Al-PMH is ∼1.32 nm, slightly thicker than that of PMH, ascribing to the disturbance of the interlayer silica framework or gallery contract upon Al-grafting under alkaline condition. The possibility of Al incorporating into the layer silica framework is precluded by the fact that the layers show smooth fringes and uniform thickness. 3.3. Texture Property. The N2 adsorption isotherms for PMH, PAMH, and xAl-PMH as well as H−K and BJH pore size distributions (PSD) plots are shown in Figure 4. The isotherms of PMH and PAMH show typical IV type, implying the presence of mesopores.33 They both have a steep linear uptake of N2 within the partial pressure p/p0 range 0.05−0.3, indicating the presence of supermicropores and small mesopores within 1.5−2.5 nm.33 The hysteresis loops can be qualified to H4 type, corresponding to uniform slit-like pores18,34 with open at both ends or spaces between parallel plates.11 Note that a wide flatter and smaller separation between desorption and adsorption branches of both PMH and PAMH implies a broad PSD plot, which is severer for PAMH than PMH owing to a slightly lower initial p/p0 (0.40) for capillary condensation of PAMH than PMH (0.45). While at the low p/p0 region, a steep linear uptake of N2 within p/p0 range 0.05−0.3, quite similar to the Langmuir-type isotherms, suggests the presence of considerable micropores. These analyses are in good agreement with the PSD plots of PMH and PAMH upon BJH model (Figure 4B(a,b)). 12229



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the previous report for Na[Al]magadiite,27 also implying the substitution of Si with Al in the layered silica framework. The 29Si MAS NMR spectra of xAl-PMH samples are quite similar to that of pure PMH. With increasing Al content, the relative proportions of Q2 sites (integral area) slightly reduce from 4.1% to 3.6%, implying the gradual consumption of silanol groups during alumination. The incorporation of Al atoms in the siliceous framework causes an increase in Q3 sites and decrease in Q4 sites as a consequence of tetrahedral [AlO4]formation. The Q3 signals broaden due to the overlap of Si(OSi)3(OAl) (at −106 ppm) on Si(OSi)3(OH),28 which makes it difficult to measure the amount of remaining silanols. Also, Q4 signals gradually broaden and slightly shift from −111.2 to −110.8 ppm, indicating the slightly increased Si(OSi)3(OAl) signal after Al-grafting probably because of the overwhelming structure stability of mesostructure silica-clay layer assembled PMH over amorphous MCM-41 or Laponitedervied PCH.30 The relative proportion of Si environment and the Q4/Q3 ratio of the framework could be evaluated from the peak area of the 29Si MAS NMR spectra.37 It can be seen (Supporting Information Table S2) that the short-range environments are clearly affected by the Al grafting routes. The decrease of Q4/Q3 ratios after Al-grafting is likely to associate with the disturbance of the chemical environment for Si−O framework caused by the incorporation of 4-coordinated Al.36 The increase in Q3 due to the overlap of Si(OSi)3(OAl) (at −106 ppm) on Si(OSi)3(OH)28 and decrease in Q4 due to the substitution of Si by Al in interlayer silica framework. Indeed, the lower Q4/Q3 ratio suggests a lower polymerization and less condensed Si−O structure in interlayer mesoporous silica,37 strongly implying the Al incorporated into the interlayer mesostructure for novel xAl-PMH materials. 27 Al MAS NMR spectra of the samples are shown in Figure 5B. The pure PMH, whose chemical composition is free of Al, shows no 27Al resonance. The xAl-PMH samples exhibit the intense lines in the range 52−55 ppm and moderate lines in the range from −0.3 to −1.9 ppm, characteristics of tetrahedral and octahedral Al coordinated sites,16,25,26 respectively, indicating the predominant Al incorporation into the interlayer silica framework with partial nonframework Al. The samples with less Al-grafting present the more tetrahedral Al sites. With increasing Al-grafting, the amount of octahedral Al sites is increased (Table 2). Indeed, an increase in concentration of NaAlO2 leads to an undesired increase of octahedral Al sites and would even cause a total destruction of the lamellar habit of PMH in strong basic solutions such as the sample 0.6Al-PMH (reaction pH 13−14). While PAMH contrarily shows

Si ratio of 0.2Al-PMH and 0.4Al-PMH seemingly indicate that the high concentration of alumination agent (NaAlO2) does not introduce more Al in intragallery silica framework but may lead to partial collapse of the meso-structure of the pristine PHM with certain Al dissolution in strong alkaline solutions, thus inducing excess micropores such as in 0.6Al-PMH as PSD revealed. These results suggest that the proper Al agent concentration plays key role in the Al postgrafting of PMH for novel high Al-containing ordered micro-mesostructured materials. Polverejan et al.16 previously reported that Alsaponite/PCH shows small change in textural porosity with lower Al contents (Si/Al = 10). We believe that the 0.4Al-PMH is the first Al-PMH sample kept well porous magadiite hetetrostructure with higher interlayer Al-grafting content. Though PAHM and PMH have close larger surface area and pore volume, they hold less or none Al in the interlayer silica framework probably render them less acidity. 3.4. Role of Aluminum. 29Si MAS NMR spectra are obtained to get further light on the response of PMH Siframework to the process of alumination. As shown in Figure 5A, except PAMH showing two distinct 29Si MAS NMR peaks,

Figure 5. Solid state 29Si (A) and 27Al (B) MAS NMR spectra for PMH (a), PAMH (b), 0.2Al-PMH (c), 0.4Al-PMH (d), and 0.6AlPMH (e).

other samples exhibit a broad peak around −111 ppm with an obvious shoulder in the high field. Therefore, each 29Si MAS NMR spectrum was carefully deconvoluted using Gaussian model. The pure PMH exhibits a prominent Q4 Si(OSi)4 resonance near −111.1 ppm, a moderate Q3 Si(OSi)3(OH) one near −102.3 ppm, and a weak Q2 Si(OSi)2(OH)2 near −93.0 ppm, according to previous NMR assignment,22,27,32,35 similar to those of MCM-41 related materials.28,36 Compared with the 29Si MAS NMR of Na-magadiite in literature,25 which exhibits three sharp Q4 signals at −109.2, −111.1, and −113.6 ppm together with one distinct Q3 peak at −99.2 ppm, the PMH shows obviously broadened Q3 at ∼−102 ppm as well as Q4 at ∼−111 ppm, also identical to the resonance of assembled gallery silica,10 confirming the transformation of TEOS into the SiO4 tetrahedra by intragallery hydrolysis and condensationpolymerization process. While PAMH shows pronounced Q3 signal despite of its lower Al content (Al/Si = 0.030), which can be ascribed to the creation of more silanol sites from the Alincorporated magadiite. This phenomenon is quite similar to

Table 2. Deconvolution of 27Al MAS NMR Spectra: Chemical Shift Values, Relative Portions, and Al/Si Molar Ratios of Different Coordinated Al Sites samples PAMH 0.2AlPMH 0.4AlPMH 0.6AlPMH a

12230

δtetra (ppm)

δocta (ppm)

Altetra (%)

Alocta (%)

Altetra/Si (102)

Alocta/Si (102)

bulk Al/ Si ratio (102)a

52.1 54.3

4.9 −1.9

42.2 68.7

57.8 31.3

1.3 4.6

1.7 2.1

3.0 6.7

53.4

−1.6

70.4

29.6

6.4

2.7

9.1

52.1

−0.3

52.8

47.2

3.5

3.1

6.6

Based on EDX data. dx.doi.org/10.1021/ie502266r | Ind. Eng. Chem. Res. 2014, 53, 12224−12237


Article

Figure 6. NH3-TPD profiles of all samples (A), deconvoluted NH3-TPD profile of 0.4Al-PMH (B), and representation of various Al sites in the xAlPMH catalysts (C).

Table 3. Acid Site Concentration and Distribution Based on NH3-TPD and Pyridine FT-IR samples

weak-to-medium (μmol/g)

strong (μmol/g)

total acidity (μmol/g)

acidity density (NH3/nm2)a

coveragesurf. by NH3 (%)b

TDesorp. (°C)

B sites (μmol/g)

L sites (μmol/g)

B/L ratio

PMH PAMH 0.2Al-PMH

170 (155)c 90 (187) 240 (201)

− 430 (530) 270 (453)

170 520 510

0.14 0.42 0.83

2.11 6.34 12.46

0.4Al-PMH

340 (214)

360 (461)

700

1.49

22.55

0.6Al-PMH

340 (226)

480 (468)

820

2.64

39.53

150 250 150 250 350 150 250 350 150 250 350

− trace 54 40 33 109 94 78 46 25 20

− 94 188 167 120 208 182 130 257 231 163

− − 0.29 0.24 0.28 0.52 0.52 0.60 0.18 0.11 0.12

a Acidity density (NH3 molecules/nm2) = [acidity (mol/g) × NA/SBET (nm2/g)]. bPercentage of the surface covered by a monolayer of chemisorbed ammonia, assuming that the surface occupied by one NH3 molecule is 0.15 nm2. cData in parentheses were the maxima desorption peak temperatures (°C) of NH3-TPD profiles.

contribution of Brønsted (B) and Lewis (L) sites by pyridine FT-IR method. The NH3-TPD results (Figure 6 and Table 3) show that the PMH has a very weak desorption at 155 °C assigned to NH3 desorbed from small amount of weak acid sites with total acidity ∼0.17 mmol/g. The desorption peak intensity is correlated with the magadiite layer intrinsic acidity that can be ascribed to the intrinsic protons created from the destruction of surfactant during the calcinations.8,10,16,30 While the PAMH shows a very weak peak at 200 °C and a strong one at 545 °C with total acidity ∼0.52 mmol/g, implying the presence of strong acid sites probably related to its considerable octahedral Al sites (see 27Al MAS NMR in Figure 5B). Then, for the xAl-PMH samples, different from montmorillonite-based Ti-PCH with one broad peak,13 the desorption of NH3 proceeds in two distinguishable temperature ranges of 100−350 °C and 350−800 °C, indicating the existence of two kinds of acid sites with varied strengths. The surface concentration of chemisorbed NH3 increases with the Al content. Meanwhile, both the peak temperatures and the relative peak intensities in both low and high temperature regions rise with the Al content, implying that both the acid sites amount and strength is increased with the Al content and total acidity of the xAl-PMH samples is 3.0−4.8 times of PMH. Note that PAMH and 0.2Al-PMH possess close total acidity though the Al content in the former is only half of the latter, indicating strong dependence of the acidity of Al-containing PMH materials on Al-coordination. Also note that PAMH shows the strongest acidity probably originated from its Na-

predominant octahedral Al sites (58%) despite of its Alcontaining host layer, which is probably due to the disordered Al-containing layered silica framework and partial dealumination of the Al-containing Si−O network during the calcinations, as evidenced by the appearance of a strong signal at 4.9 ppm (octahedral Al sites), in accordance with its 29Si MAS NMR data. These findings clearly suggest that the varied Al incorporation routes result in varied Al, Si-coordination environments, and the postsynthesis alumination method, as a better way than Al-magadiite precursor method, can effectively realize layered supermicro−mesostructure with substitutional doping of the interlayer [SiO4] framework of PMH with Al upon both the thermal stable PMH and the proper amount of aluminate. Occelli et al.38 previously reported that the dealumination increases upon decreased SiO2/Al2O3 ratio in direct synthesis of MCM-41-type materials and thus greatly reduced tetrahedral Al sites, similar to more early work by Kosslick et al.39 Zimowska et al.30 recently published a more uniform distribution of Al sites obtained by using inorganic Al source for the alumination process in laponite-derived PCHs. In this work, we find that the xAl-PMH (x = 0.2, 0.4) samples possess more structurally uniform tetrahedral Al sites by postgrafting route with the use of NaAlO2 than PAMH from Na[Al]magadiite, suggesting the possible prevention effect on dealumination from the interlayer silica network due to the protection of the PMH layers. 3.5. Surface Acidity. The surface acid sites concentration and strength were determined by NH3-TPD, while the 12231



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Figure 7. FT-IR spectra of pyridine adsorbed on PMH (a), PAMH (b), 0.2Al-PMH (c), 0.4Al-PMH (d), and 0.6Al-PMH (e) at elevated temperatures: 150, 250, and 350 °C.

Pyridine in situ FT-IR spectra were used to further probe the nature of the acidity of the PMH-based catalysts. For all the samples, the FT-IR spectra are obtained (Figure 7) in the region 1750−1400 cm−1 and the number of both B and L sites obtained from the integrated area of IR bands at 1545 cm−1 (B) and 1452 cm−1 (L) with extinction coefficient 0.73 and 1.11 cm·mol−1,43,44 respectively. As seen in Figure 7a, the PMH only shows the bands at 1445 and 1593 cm−1, assigned to pyridine adsorbed on weak L sites or to weakly hydrogen-bonded pyridine,30,44,45 which are nearly vanished after outgassing at 250 °C, thus indicating the existence of very weak acid sites on PMH. While the PAMH (Figure 7b) shows obviously enhanced bands at 1445 and 1593 cm−1 upon pyridine adsorbed on strengthened L sites or to weakly hydrogen-bonded pyridine.30 Moreover, two new weak bands at 1545 and 1490 cm−1 can be observed, attributable to pyridine bonded to B sites (pyridinium ion) and a combined contribution from both B and L sites,30,44 implying the formation of very small amount of B sites and increased L sites due to the Al incorporation into the layer silica framework from [Al]magadiite. After outgassing at 250 °C, the intensities of these bands are greatly reduced, more pronounced for the L sites, indicating the presence of weak L and B sites on PAMH. Upon Al-grafting, the xAl-PMH samples (Figure 7c,d,e) present clearly enhanced absorptions at 1445, 1490, 1545, and 1593 cm−1 related to both B and L sites. Compared to PAMH, the band at 1445 cm−1 becomes asymmetric and broad with a weak distinguishable left shoulder near 1452 cm−1, implying greatly increased L sites besides the weakly hydrogen-bonded pyridine, and the clear new band at 1619 cm−1 (L) also strongly support this point. More importantly, the greatly enhanced band at 1545 cm−1 together with a new one at 1638 cm−1 undoubtedly indicates the increased B sites of postgrafting samples. The FT-IR of the xAl-PMH samples upon outgassing at high temperature (250 °C) show clear bands at 1452 cm−1 with obviously reduced intensity and other bands with almost unchanged position and intensity, further supporting the presence of strong L and B sites, in line with above NH3-

[Al]magadiite precursor with Al-doped layers and considerable dealuminated extra-framework Al in this sample (Figure 5B, 27 Al NMR). The present results reveal that the postgrafting route can markedly enhance the total surface acidity of siliceous PMH by creating more moderate and strong acid sites due to the more uniformly incorporated Al centers in interlayer mesostructured silica. These findings are in good agreement with the amounts of tetrahedral Al sites as 27Al NMR judged. It should be noted that chemisorbed NH3 (assuming 0.15 nm2 occupied per NH3 molecule40) covers the fraction of the sample surface lower than 23% except poor-crystallized 0.6AlPMH assuming the formation of monolayer (Table 3). We further attempt to deconvolute desorption peaks in NH3TPD of the PMH-derived catalysts using Gaussian function with temperature as variant according to previous reports on HAlMCM-41.41,42 The first three peaks are assigned to weak, moderate, and strong B acid sites, respectively.41 The weak acid sites at ∼169 °C are assigned to surface hydroxyl groups, while two other peaks at 212 and 281 °C originate from moderate and strong structural acid sites (B) due to the presence of Al3+ in the silica framework. The broad peak at 427 °C may arise from tricoordinated Al3+ (weak L sites) and additional even broad peak at 556 °C to nonframework Al3+ (strong L sites) generated during the calcinations. It can be seen that the area under profile corresponding to the moderate and strong B sites is comparable to that of strong L sites. A representation of the varied acid sites is shown in Figure 6C. Clearly, the framework Al is responsible for the medium-to-strong B sites, which is useful for chosen reaction, while the extra-framework Al is responsible for L sites. The xAl-PMH samples with Al grafted into tetrahedral silica framework afford to medium-to-strong B sites (ii and iii) and structural weak L sites (iv), and with small amount of nonframework Al afford to strong L sites (v), consistent with 27Al MAS NMR data. However, the PAMH from Na-[Al]magadiite with Al in the layers affords to only weak L sites (iv) and strong L sites (v), further indicating the key role of the localization and coordination environment of Al on the acidity of the PMH-based catalysts. 12232



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Figure 8. Effects of reaction conditions on CAT conversion and products selectivity over 0.4Al-PMH with 10 wt % catalyst loading. (a) Reaction temperature (TBA/CAT = 2.0, 4 h), (b) TBA/CAT molar ratio (138 °C, 4 h), and (c) reaction time (TBA/CAT = 2.0, 138 °C).

newly formed B acidity of xAl-PMH (x = 0.2, 0.4) are highly desirable for extended acid catalytic applications. It is noted that 0.4Al-PMH shows the most and strongest B sites among all the samples. Clearly, the Al-modified PMH-based catalysts developed from varied synthesis routes, that is, PAMH from Na-[Al]magadiite and xAl-PMH from postgrafting Al, exhibit totally different acidity, which is intimately related to the Alincorporating positions and Al coordinated forms, thus opening a rational opportunity for selective catalysis applications over desired solid acid materials. 3.6. Catalytic Performance. The alkylation of catechol (CAT) with tert-butyl alcohol (TBA), a typical Friedel−Crafts acid-catalyzed electrophilic substitution reaction, is an important industrial reaction because its monoalkylated product 4tert-butylcatechol (4-TBC) and related derivatives are often used as effective antioxidants, stabilizers and polymerization inhibitors for many organic compounds.49−52 Therefore, the liquid phase Friedel−Crafts tert-butylation of CAT is used to evaluate the catalytic performance of the PMH-based catalysts. The main product 4-TBC is separated and identified by 1H NMR spectrum showing δ 6.93 (singlet, 1H, ArH), δ 6.81 (multiplet, 2H, ArH), δ 5.34 (broad multiplet, 2H, OH), and δ 1.27 (singlet, 9H, t-Bu) (Supporting Information Figure S6). The sample 0.4-Al-PMH with well-kept supermirco− mesopore structure, high surface area, and high acidity is employed to systematically investigate the effect of reaction temperature, molar ratios of reactants, and reaction time on CAT tert-butylation activity for better synthesis of 4-TBC. The effect of temperature (Figure 8a) shows that at TBA/ CAT ratio of 2.0 and reaction time of 4 h, the CAT conversion increases generally with temperatures (110−138 °C), however, the 4-TBC selectivity (abbreviated Sel.) decreases accompanying with the increased Sel. of 3,5-DTBC. Though the Sel. of 4TBC is as high as 91.8% at 110 °C, the CAT conversion and 4TBC yield are rather low. While the 4-TBC yield exhibits an identical trend as CAT conversion with temperature, reaching the highest value 75.1% at 138 °C. Thus, the temperature 138 °C is chosen as an optimal temperature given the boiling point 138.5 °C of p-xylene. The effect of TBA/CAT ratio (Figure 8b) reveals that the CAT conversion obviously increases from 83.6% to 95.7% with increasing TBA/CAT ratio (1.0−3.0), while the 4-TBC Sel. shows sequential decrease as 87.2%, 80.4%, and 68.7% with TBA/CAT ratio of 1.0, 2.0, and 3.0, respectively, correspondingly with linearly increased Sel. of 3,5DTBC. The increased 3,5-DTBC Sel. can be ascribed to the availability of excess TBA as the more TBA in the reaction

TPD data. With further enhanced desorption temperature (350 °C), the intensities of L sites within 1460−1440 cm−1 is slightly decreased and broadened, seemingly indicating that the pyridine may bound to two kinds of L sites, also in line with the NH3-TPD results. While the bands corresponding to B sites at 1545 cm−1 is obviously reduced but still observed at 350 °C. The 0.4Al-PMH holds considerable amount of B and L sites even at 350 °C, revealing the most and strongest acidity of this sample. The detection of B sites and two kinds of L sites with varied strengths and distributions as a function of temperature could be attributed to the clay-layer silanol groups or the interlayer mesostructure silica. Similar phenomena was found in montmorillonite, saponite, and laponite-derived PCH materials.14,15,30 In order to get more precise concentration of B and L sites, the band at ∼1452 cm−1 is carefully deconvoluted by Gaussian model (insets in Figure 7) to discriminate between the coordinated pyridine to L sites and weakly hydrogen-bonded pyridine to silanol groups. The amount of both L and B sites (Table 3) are obviously affected by the nature of PMH and the synthesis strategy. Upon postgrafting, the amount of both L and B sites is significantly enhanced compared to PAMH from Na-[Al]magadiite. The relative proportion of B to L sites indicates that the L acidity dominates the surface of PMH derived materials. Note that the total acidity from pyridine FTIR is obviously lower than those from NH3-TPD probably due to the larger dynamic diameter of pyridine (0.58 nm) than NH3 (0.26 nm)46 preventing pyridine from adsorbing on the surface of micropores and the stronger affinity of pyridine to strong acid sites.30 It is well-known that pure MCM-41 is Lewis solids without B sites,47,48 and the present PMH does not possess B sites; therefore, the B sites of the xAl-PMH samples are caused by the presence of Al atoms replacing the Si ones. Given much larger radius (0.039 nm) of Al3+ ions than Si4+ (0.026 nm), when the smaller Si4+ ions are replaced by the larger Al3+ ions in interlayer silica framework of the solid, the bond length of Al− O−Si clearly differs from that of Si−O−Si, and this phenomenon must lead to structural microstrain within the lattice cell. Changes in the electron density around Si, due to charge unbalance or differences in electronegativity or local structure deformation owing to the introduction of Al3+ into the vicinity of the hydroxyls carrying Si, may weaken the SiO− H bond; this is one of the possible origins giving rise to the B sites via bridging Al-(OH)-Si groups expected to form on the Al-grafted PMH materials. The greatly enhanced L acidity and 12233



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Table 4. Catalytic Performance of the Catalysts for Alkylation of Catechol with tert-Butyl Alchola Sel. (%) catalysts

conv. (%)

4-TBC

3-TBC

3,5-DTBC

yield4‑TBC (%)

PMH PAMH 0.2Al-PMH 0.4Al-PMH 0.6Al-PMH AlCl3 H2SO4 Si-MMTb Si/Al-MMTb 15WOx/ZrO2c SO3H-ionic liquidd

22.40 33.10 95.81 93.44 92.40 19.20 59.93 68.1 76.8 99.0 41.5

2.15 25.48 73.27 80.42 36.34 89.20 91.80 77.6 85.2 73.0 97.1

97.18 57.83 7.89 0.63 52.54 8.72 2.02 10.8 8.6 − 2.2

0.66 16.69 18.84 18.95 11.12 2.10 7.18 11.6 6.2 27.0 0.7

0.48 8.43 70.20 75.14 33.58 17.13 55.02 52.85 65.43 72.27 40.30

refs this this this this this this this 12 12 53 52

work work work work work work work

CAT/TBA molar ratio = 1:2, 9 mL p-xylene, 138 °C, 4 h, 0.22 g catalyst (10 wt % of CAT). bCAT/TBA = 1:2, m-xylene, 135 °C, 8 h, 0.22 g catalyst in ref 12. cCAT/TBA = 1:2, 140 °C, 0.5 h. Catalyst loading: 15 wt % of CAT in ref 53. dCAT/TBA = 2:1, 150 °C, 3 h, 0.293 g catalyst (5 mol % of CAT) in ref 52. a

liquid52 under similar experimental conditions, even comparable or slightly higher than those of HZSM-5, H-USY, and Hβ50 though slightly lower than recently reported solid superacid WOx/ZrO2.53 The xAl-PMH catalysts are verified to possess an open gallery framework with pores (>1.6 nm) much larger than microporous zeolites (i.e., H-ZSM-5 with 0.54 × 0.56 nm54), enabling a much easier diffusion of reactant or product. Moreover, the acidities of xAl-PMH are comparable to commonly studied zeolites.55 Both factors favor a high activity of the xAl-PMH samples. The surface acidity of the catalysts is also shown to greatly influence the products distribution. Among all the samples, PMH gives the lowest 4-TBC Sel., the highest 3-TBC Sel., and only trace 3,5-DTBC. It has been reported that 3-TBC formation favors weak acid sites,50 so it is reasonable for PMH with very weak acidity showing as high as 97.2% Sel. for 3-TBC. While PAMH shows 23.3% increase in 4-TBC Sel. and 40.6% decrease in 3-TBC Sel. attributable to the enhanced acid amount and strength due to the Al incorporation in the layer silica framework, compared to PMH, but 3,5-DTBC Sel. is obviously enhanced to 16.4%, implying that the dialkylated product formation favors strong acid sites. Then, for the xAlPMH samples, the 4-TBC Sel. is substantially higher than PMH and PAMH depending on the B acid amount and strength (Table 3). Specially, 0.4Al-PMH gives the highest 4-TBC Sel. owing to its highest B acid amount and strength. However, the 3-TBC Sel. and 3,5-DTBC Sel. is not proportional to the acid concentration or strength in any case, implying the presence of other factors affecting the product distribution besides the surface acidity. The Sel. of 3,5-DTBC with larger size (0.92 × 0.66 nm, based on ChemDraw Ultra 8.0) reduces with an order of 0.4AlPMH ≈ 0.2Al-PMH > 0.6Al-PMH, in good agreement with their nonmicropore surface and supermicro−mesopore structure (Table 1), considering the steric diffusion hindrance of 3,5DTBC produced from further reaction of 4-TBC/3-TBC with excess TBA. Yoo et al.50 reported that the formation of 3,5DTBC occurred on external surface of zeolites. Interestingly, 0.4Al-PMH gives the lowest 3-TBC Sel. 0.6%, close to 0.5% of HY,51 probably due to the smaller size of 3-TBC (0.76 × 0.54 nm) compared to 4-TBC (0.82 × 0.54 nm), thus smaller diffusion hindrance in micropores. Upon the experimental facts, it can be deduced that 3-TBC Sel. is likely proportional to microporous surface area. The smallest microporous area of

system, the higher the possibility of unreacted TBA and monoalkylated products to collide at the active sites, leading to the formation of more 3,5-DTBC.53 The 4-TBC yield varies with increasing TBA/CAT ratios and reaches the highest at TBA/CAT = 2.0. Hence, the suitable TBA/CAT ratio is 2.0. The effect of reaction time (Figure 8c) displays that the CAT conversion nearly linearly increases from 33.3% to 93.4% within 1−4 h, and then remains almost constant up to 8 h. The Sel. of 4-TBC and 3-TBC reaches 58.4% and 41.6% at 1 h, respectively, which can be ascribed to initial competitive formation of these two products in supermicro−mesostructured 0.4Al-PMH. However, the 4-TBC Sel. rapidly increases to 88.1% h at 2 h, and slightly reduces to 80.4% at 4 h and keeps constant until 8 h, while the 3-TBC Sel. dramatically reduces to 7.1% at 2 h and 0.63% at 4 h, probably due to the higher thermodynamic stability and kinetic preference of 4-TBC over 3-TBC.52 As for the formation of 3,5-DTBC (18%), it is produced by further alkylation of 4-TBC or 3-TBC in mesoporous catalyst with moderate-to-strong acid sites.53 Thus, the reaction time of 4 h is enough to reach equilibrium in the present study. Then, the catalytic performance of all PMH-based catalysts is evaluated at optimal conditions (138 °C, TBA/CAT ratio of 2.0, 4 h). As shown in Table 4, the CAT conversion shows considerable differences upon the surface acidity of the catalysts. The xAl-PMH samples clearly exhibit much higher activity, particularly above 92% CAT conversion, compared to PMH and PAMH. However, with reduced surface area and pore volume of xAl-PMH, the conversion drops slightly. Noteworthy, though PAMH and 0.2Al-PMH hold identical total acidity upon NH3-TPD (Table 3), the CAT conversion of 0.2Al-PMH is nearly 2.9 times of PAMH. From above pyridine FT-IR, on outgassing at 250 °C, the amount of B and L sites for 0.2-Al-PMH are 40 and 167 μmol/g, respectively, while PAMH holds only L sites (94 μmol/g). From the fact that 0.2Al-PMH exhibits remarkably better CAT conversion, it could be speculated that the enhanced acidity in interlayer mesostructured silica framework, especially B acidity, is the decisive factor determining the catalytic properties. Similar phenomena are observed by Yoo et al.50 over several zeolites. The present CAT conversions over xAl-PMH samples are evidently higher than conventional H2SO4 (B) and AlCl3 (L) (Table 4) and previously reported HY zeolite,51 γ-Al2O3,50 Si−Al montmorillonite heterostructures,12 and SO3H-functionalized ionic 12234



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and stable supermicro−mesostructure. The xAl-PMH catalysts show greatly improved liquid-phase tert-butylation activity of catechol to shape-selectively produce 4-tert-butylcatechol with CAT conversions larger than 92%, far above PMH (22%), PAMH (33%), and most previously reported values under similar reaction conditions. The sample 0.4Al-PMH presents the largest 4-TBC selectivity (80.4%) and yield (75.1%) due to its strongest synergy among the total acidity, the B/L acidity ratio, and the layered supermicro−mesoporous structure intimately associated with the Al-grafting amount, Al coordination forms, and Al substitution positions (layer or interlayer silica framework). The present work provides the possibility for tuning surface acidity and texture property of the porous clay heterostructures upon postgrafting strategy for a large variety of acid-catalyzed processes of important fine chemicals.

0.4Al-PMH rationally afford to its lowest 3-TBC Sel., while the extremely higher 3-TBC Sel. of 0.6-Al-PMH (52.5%) is likely due to the larger micropore surface from collapsed layer structure and decrement of B sites. Clearly, the 4-TBC Sel. is determined by both the B acidity and the porosity of the catalysts. Meanwhile, 0.4Al-PMH exhibits the highest yield for 4-TBC (75.1%), substantially superior to traditional B acid H2SO4 or L acid AlCl3, even comparable to commercial H-ZSM-5 with similar Si/Al ratio,50 owing to its strongest B acidity and unique supermicro−mesoporous structure. Moreover, the repeated uses of 0.4Al-PMH (Supporting Information Table S3) show 92.2% CAT conversion and 80.2% 4-TBC Sel. in the first cycle, almost same as the fresh catalyst, and essentially constant CAT conversion and 4-TBC Sel. in the followed two cycles. From the above results, postgrafting samples xAl-PMH have a potential to become a favorable acid catalyst for the tertbutylation reaction, because of their excellent activity with high selectivity, much lower corrosion tendency, excellent thermal stability, and good recyclability. The sample 0.4Al-PMH exhibits the best alkylation performance among all the samples, indicating a strong influence of the Si−OH-Al groups related to isomorphous tetrahedral Al substitutions for Si. Based on the XRD, BET, 27Al and 29Si MAS NMR, NH3-TPD, and pyridine FT-IR data, the relationship between the tert-butylation activity and the nature of the PMH-based catalysts is proposed and presented in Scheme 2.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 showing the TG results of starting material and intermediates. Figure S2 and S3 showing the XRD patterns and FTIR spectra in each step of PMH and PAMH formation, respectively. Figure S4 and S5 showing SEM, TEM, and HRTEM images in each step of PMH and PAMH formation, respectively. Figure S6 showing 1H NMR of 4-TBC. Table S1 showing chemical compositions of the samples. Table S2 showing the deconvolution results of 29Si MAS NMR of the catalysts. Table S3 showing the recyclability of 0.4Al-PMH. This material is available free of charge via the Internet at http://pubs.acs.org/.

Scheme 2. Effects of the Acidity and Porosity of the Supermicro−Mesostructed xAl-PMH Catalysts on the Alkylation of Catechol (CAT) with tert-Butyl Alchol (TBA)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +8610-6442 5872. Fax: +8610-6442 5385. Email: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CBA00508), the National Natural Science Foundation of China (21276015), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1205).

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REFERENCES

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Aluminated Derivatives of Porous Magadiite Heterostructures for Acid

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