Facile tailoring of hierarchical mesoporous AlSBA-15 by ionic liquid and their applications in heterogeneous catalysis

Significant enhancement in the performance of incorporation of high content of aluminum within hierarchical mesoporous SBA-15 has been achieved by direct route using urea tetrachloroaluminate ionic liquid as novel aluminum source. The fabricated materials were fully characterized by N₂ sorption isotherms, powder X-ray diffraction (XRD), FT-IR, ²⁷Al MAS NMR, XRF, HRTEM and FESEM. The acidic properties of these materials have been examined using NH₃-TPD. The catalytic performance was investigated using cumene cracking and the esterification reaction as a probe molecules to estimate the acidity of the material. It is worth noting that the ionic liquid with accomplished Al–O–Si bonds is an efficient precursor to synthesis AlSBA-15 with high aluminum content (nSi/mAl?=?7) without destroying the structural order of the material in acidic medium. ²⁷Al MAS NMR spectra of AlSBA-15 show that all aluminum species were incorporated into the SBA-15 framework with nSi/mAl ratio up to 7. Overall, this work emphasize that the AlSBA-15 materials contain Bronsted and Lewis acid sites with medium acidity which makes them adequate to be adopted as acid catalysts in heterogeneous catalysis.     

Synthesis and characterization of W-SBA-15 and W-HMS as supports for HDS

W-modified HMS and SBA-15 mesoporous materials (Si/W molar ratio equal to 40) were synthesized using sodium tungstate as tungsten source. In order to prepare NiW catalysts these mesoporous materials were impregnated with an aqueous solution of nickel salt of 12-tungstophosphoric acid Ni_3/2PW₁₂O₄₀. The synthesized W-HMS, W-SBA-15 materials and NiW catalysts have been characterized by S_BET, XRD, UV–Vis DRS, FT-IR, TPD of NH₃, ²⁹Si MAS NMR, SEM and HRTEM. The influence of these particular supports on catalytic activity of NiW catalysts was studied in the reaction of hydrodesulfurization (HDS) of thiophene. The results from the FT-IR and UV–Vis DR spectroscopy confirm incorporation of W into the HMS and SBA-15 structures. Additionally ²⁹Si MAS NMR measurements revealed relatively stronger effect of W ion incorporation in HMS structure on degree of silica cross-linking as compared to the effect of W ion incorporation in SBA-15 structure. The catalytic study showed that both W-HMS and W-SBA-15 materials modified with W are good supports for NiW catalysts in the HDS reaction of thiophene. The catalysts show lower selectivity for butanes than a reference NiW/γ-Al₂O₃ catalyst leveling of about 10% for chosen experimental conditions.     

Effect of different synthesis methods on the structural and catalytic performance of SBA-15 modified by aluminum

Two different aluminum-containing mesoporous Al/SBA-15 were prepared by post-synthesis method and direct synthesis method using P123 as structure directing agent and aluminum isopropoxide as the aluminum source. Samples were characterized in detail by XRD, N₂ adsorption, TEM,²⁷Al MAS NMR, and the acidic properties were performed by FT-IR of pyridine adsorption (Py-IR). The results show that Al/SBA-15 prepared by the direct synthesis method possessed higher BET surface area and larger pore volume. The acidic properties were investigated in Friedel–Crafts alkylation of phenol tert-butylation. The different synthesis method leads to distinct coordination of Al and intensity of acidity. Kinetics of the alkylation over Al/SBA-15 was also investigated. These results indicated that Al/SBA-15, prepared by post-synthetic grafting method, has more activity ability than the corresponding one synthesized by direct method since they contain more Lewis acid sites and Brönsted acid sites. The catalysts preserved almost their initial catalytic activity after four reuses.     

Effect of acid type in WO X clusters on the esterification of ethanol with acetic acid

Tungsten oxide clusters supported on silica (WO _X /SiO₂) with different W loading levels and the effect of acid type on the esterification of acetic acid with ethanol were examined. The catalysts were characterized using various techniques (XRD, Raman spectroscopy, NH₃-TPD and FT-IR) to investigate the crystallinity and the nature of the acid sites. The change in the composition of two tungsten oxide species (polytungstate and crystalline WO₃) leads to the change of Lewis acid to Br?nsted acid ratio. Importantly, the ratio of the two different acid types has a substantial effect on the catalytic activity. The fraction of Lewis acid to total acid sites rapidly changed from 23% to 77% due to the presence of crystalline WO₃. Where the Lewis acid sites accounted for 55% of the total acid sites, the WO _X /SiO₂ catalyst showed the highest catalytic activity among the prepared catalysts.     

Direct synthesis,characterization and catalytic performance of Al–Fe-SBA-15 materials in selective catalytic reduction of NO with NH3

A series of Al–Fe-SBA-15 catalysts were synthesized by microwave methods with various iron and aluminum contents. The samples were characterized by XRD, SEM, TEM, BET, NH₃-TPD, FT-IR. Results indicated that all the samples that exhibited well-ordered hexagonal arrays of mesopores, disk-like and petal-like morphologies were observed for different catalysts. NH₃-TPD indicated that the introduction of Al results in the generation of Brönsted acid originated from Al–OH. Brönsted acid played important roles in the selective catalytic reduction of NO by NH₃, NO_x conversion over Al–Fe-SBA-15 samples increased rapidly as temperature increased and achieved 95% at about 360 °C.     

Effect of Treatment with NaAlO2 Solution on the Surface Acid Properties of Zeolite β

The influence of treatment with an NaAlO₂ aqueous solution of NaAlO₂ on the surface acid properties of zeolite β was studied as a method to increase the number of both Brønsted and Lewis acid sites. The Hβ samples, prepared by treating with an aqueous solution of NaAlO₂, were characterized by XRD, NH₃-TPD, pyridine-IR, ²⁹Si MAS NMR, and ²⁷Al MAS NMR. The surface properties of the Hβ zeolite changed after treatment with the aqueous NaAlO₂ solution: the number of strong and weak acid sites, the total amount of acid, and the Brønsted and Lewis acid sites increased. The treatment of zeolite β with an aqueous solution of NaAlO₂ may realuminate an Si(0Al)_A site by the isomorphous substitution of Al(OH)₄⁻ anions.     

Enhanced performance of methane dehydro-aromatization on Mo-based HZSM-5 zeolite pretreated by NH4F

Enhanced performance of methane dehydro-aromatization reaction (MDA) were achieved on a Mo-based HZSM-5 zeolite catalyst in which HZSM-5 were pretreated by a proper amount of NH₄F (Mo/HZ(F)). The results of NH₃-TPD and ²⁷Al MAS NMR demonstrated that the number of Brönsted acid sites decreased on the HZSM-5 zeolite and Mo/HZSM-5 catalyst after NH₄F treatment. TGA and TPO measurements showed that the Mo/HZ(F) catalysts were highly resistant to coke deposition, which resulted mainly from the elimination of the Brönsted acid sites after the pretreatment of the HZSM-5 zeolite with NH₄F.     

High-surface-area SBA-15–SO3H with enhanced catalytic activity by the addition of poly(ethylene glycol)

The influence of poly(ethylene glycol) (PEG) and synthesis temperature in the synthesis of SBA-15–SO₃H was investigated to evaluate the catalytic activity in the esterification of propionic acid with methanol. The catalysts were characterized by means of surface and structure analyses; X-ray diffraction, FT-IR, scanning electron microscopy, Thermo-gravimetric and N₂ adsorption/desorption techniques. It was found that, by the addition of PEG, the surface area and porosity of SBA-15–SO₃H increased, while the structure and size of mesopores remained unchanged. Nitrogen sorption measurements indicate that PEG introduces additional pores into the pore walls of SBA-15–SO₃H. Thus, a simple way of improving the porosity of mesoporous SBA-15–SO₃H was presented that could enhance transport of substrates through the porous system and allow the generation of stable mesoporous replicas, important for catalytic applications and also beneficial for replication and nanocasting purposes.     

Preparation of bimodal mesoporous silica containing cerious salt and its application as catalyst for the synthesis of biodiesel by esterification

A kind of bimodal mesoporous silica catalyst modified with ammonium cerous sulfate (ACS/BMMS) was synthesized and applied in the esterification of free fatty acid and alcohol. The characterization results including XRD, N₂ adsorption and desorption, FTIR, ²⁹Si-NMR and TEM showed that ACS/BMMS has orderly arranged bimodal mesopores, the small mesopore diameter is about 4.0–6.0 nm and the large mesopore diameter is in the range of 7.0–9.0 nm. The chemical interaction existed between silica group Si (OH)₂(OSi)₂ and the NH₄ ⁺, SO₄ ^2? and Ce-O groups of cerious salt. When the loading is not more than 10%, cerious salt dispersed finely on the supports. Oleic acid and methanol were used as the raw material of probe reaction; ACS/BMMS had significantly better activity than the ACS/SBA-15, ACS/SBA-16, ACS/MCM-41, BMMS and bulk ACS. The optimum loading of ACS is 10%, the optimum reaction conditions are reaction temperature 140 °C, reaction time 2 h, mole ratio of methanol to oleic acid 2.0 and the dosage of catalyst 4.0%, in above situation the conversion of oleic acid is about 94.0%, the reusability of ACS/BMMS is much better than bulk ACS. The kinetic study showed that the esterification of oleic acid and methanol on ACS/BMMS match Eley–Rideal model very well.     

B–SBA-15–SO<Subscript>3</Subscript>H: a versatile mesoporous catalyst

Novel approach was developed towards the synthesis of heterogeneous mesoporous B–SBA-15–SO₃H acid catalysts. That is: B–SBA-15 materials were hydrothermally synthesized by using three different boron sources (boric acid, trisiopropylborate and potassium borohydride) and then post-synthetically functionalized with sulfonic acid to test their catalytic activity for the esterification of propionic acid with methanol. Mesoporous and amorphous character of the materials as well as the incorporation of the boron into the framework were verified by XRD, N₂-adsorption/desorption, ICP–OES, SEM and FT-IR techniques. All of the boron incorporated acid catalysts showed better activity than pure SBA-15–SO₃H for methyl propionate synthesis. Combining the advantageous of hydrothermal synthesis and post functionalization, the obtained B–SBA-15–SO₃H catalysts should present versatile catalytic properties in terms of both catalytic activity and reusability. Boron incorporation and post functionalization encourage the structural configuration, since the grafted catalytically active groups may experience similar environments and be isolated from each other.     

Synthesis of mesoporous aluminosilicates with low Si/Al ratios using a single-source molecular precursor under acidic conditions

A single molecular precursor, bis(sec-butoxy)-aluminoxy-triethoxysilane, was used as aluminum source for the synthesis of mesoporous aluminosilicates with Si/Al ratios ranging from 1 to 10 under acidic conditions. Aluminum can be stoichiometrically incorporated into the mesoporous materials at pH = 1.5. The mesoporous aluminosilicates synthesized with the single molecular precursor display larger unit cell parameter and pore diameter than that of the materials prepared with aluminum sulfate, aluminum nitrate, aluminum hydroxide, and aluminum isopropoxide as the aluminum sources. IR spectra of adsorbed pyridine and NH₃-TPD showed that both Br?nsted acidic sites and Lewis acid sites with medium strength are present in the materials. However, no direct relationship between the acidic properties and the content of aluminum was observed when the Si/Al ratios of the mesoporous aluminosilicates are lower than 10.     

Efficient cyclohexyl acrylate production by direct addition of acrylic acid and cyclohexene over SBA-15-SO3H

A set of propysulfonic acid modified mesostructured silica materials (SBA-15-SO₃H) were prepared by a convenient one-pot co-condensation method. Samples were characterized by XRD, FT-IR, Py-FTIR, N₂ adsorption–desorption, elemental analysis and acid–base titration. The catalysts exhibited excellent activities and selectivities for addition esterification of acrylic acid and cyclohexene. The highest catalytic activity (82.8 %) and selectivity (92.6 %) were obtained over SBA-15-SO₃H (15 %). The efficient catalytic performance of SBA-15-SO₃H (15 %) was attributed to the high surface area, proper mesopore texture and presence of sulfonic acid groups in its structure. SBA-15-SO₃H catalyst was easily separated and recycled with high stability.     

Catalytic property of Pt/AlSBA-15 in selective catalytic reduction of NO

Pt-supported on mesoporous SBA-15 catalysts (Pt/SBA-15) have been investigated for selective catalytic reduction of NO by propene in excess oxygen. Catalytic activity of Pt/SBA-15 can be improved by the post-synthesis modification of Al into SBA-15. The resulting material (AlSBA-15) retains a hexagonal order and physical properties of the parent SBA-15 and shows new acid sites. Increased acid sites are confirmed by NH₃-TPD. Medium acid site appeared at 200 °C and broad strong acid site appeared above 250 °C. A certain degree of support acidity of Pt/AlSBA-15 increased the NO conversion, but when there are too many acid sites, carbon deposition becomes extensive and lead to decrease the NO conversion. The influence of different Pt loadings on the catalytic properties was also investigated.     

Highly Active SO4 2−/xTiO2–ZrO2 Catalysts for the Esterification Between Terephthalic Acid and 1,3-Propanediol

Poly (trimethylene terephthalate) (PTT) is considered as one of the excellent fiber materials in the twenty-first century. It was synthesized as early as in 1940s, but up to now, it is rather difficult to obtain the PTT with high molecular weight suitable for spinning, due to the absence of highly active catalysts. In this work, two series of catalysts, i.e., xTiO₂–ZrO₂ (x = 6, 8, 10, 15) and SO₄ ^2?/xTiO₂–ZrO₂ (x = 6, 8, 10, 15) were prepared, and used for the esterification of terephthalic acid (TPA) and 1,3-Propanediol (PDO). The synthesized catalysts were fully characterized by scanning electron microscope (SEM), infrared diffusion reflectance spectrum (IDRS), X-ray diffraction (XRD) and NH₃-temperature programmed desorption (NH₃-TPD) techniques. Based on the difference in acidities between xTiO₂–ZrO₂ and SO₄ ^2?/xTiO₂–ZrO₂ as indicated by characteristic results of IDRS and NH₃-TPD, SO₄ ^2?/xTiO₂–ZrO₂ have more Brønsted acid sites on their surfaces than xTiO₂–ZrO₂, as a result, SO₄ ^2?/xTiO₂–ZrO₂ had high activities for the esterification of TPA and PDO.     

Infrared investigations of arenesulfonic modified mesoporous materials by the use of complementary probes

Arenesulfonic mesoporous materials [SBA(R), 0.05????R (incorporation ratio) ??0.15] have been prepared by the co-condensation of tetraethoxysilane (TEOS) and 2-(4-chlorosulfonylphenyl) ethyltrimethoxysilane (CSPTMS) in acidic solutions of Pluronic P123. The extent of the surface functionalisation and the nature of the interactions between sulfonic groups and the surface in SBA(0.05) and SBA(0.15) samples were investigated by FT-IR spectroscopy using two probes of different basic strengths. It was shown that carbon monoxide (CO, at 77?K) can be used as a good probe to study the surface functionalisation through the ??_C=O vibration at 2,158?cm^?1 corresponding to its interaction with free silanol groups. Ammonia (NH₃, at 423?K) is able to disturb the strong interactions of Si?COH?CH₃OS by forming the ??_NH?CNH₄ ⁺ which allowed to emphasise that SO₃H groups of the less functionalised material SBA(0.05) are less acidic since the number of the acid groups is less important than in SBA(0.15).     

Beneficial role of SiO2 plugging in AlCl3 heterogenized plugged hexagonal templated silica (PHTS) for the isopropylation of m-cresol

Plugged hexagonal templated silica (PHTS – a modified SBA-15) has been synthesized using triblock co-polymer as template and tetraethyl orthosilicate as silica source under hydrothermal conditions and AlCl₃ has been heterogenized on it which is proven to be an efficient heterogeneous catalyst for the isopropylation of m-cresol. The activity of AlCl₃ heterogenized on SBA-15 has also been tested for the purpose of comparison. The higher activity of Al–PHTS over Al–SBA-15 has been attributed to the presence of more number of medium strength acid sites formed by the plugging of silica and higher AlCl₃ heterogenization in the former catalyst as observed from ²⁷Al MAS NMR and temperature programmed desorption of NH₃.     

Mesoporous silicas containing carboxylic acid: Preparation,thermal degradation,and catalytic performance

The calcination and thermal degradation behaviors of surfactants in mesoporous silicas SBA-15 and MCM-41 were investigated by FT-IR, ¹³C CP/MAS NMR, TG/DTA, and GPC. It was found that carboxylic acid-containing products were generated as active components in the mesopores of SBA-15 and MCM-41 from the triblock copolymer (PEO)₂₀(PPO)₇₀(PEO)₂₀ and cetyltrimethylammonium bromide (CTAB), respectively; the latter materials were used as templates. The carboxylic acid-containing mesoporous silica obtained showed a catalytic activity for hydrolysis of sucrose. The acidity was evaluated by means of NaOH titration. The acidity sensitively depended on both the calcination temperature and the atmosphere; the maximum appeared at 150 °C in air for SBA-15 where the highest activity was observed. However, the product in MCM-41 showed a lower catalytic activity than that in SBA-15. The SBA-15 product was easily leached from the mesopores of SBA-15 into the solution, but the degree of leaching for MCM-41 was considerably smaller than that for SBA-15.     

Convenient solvent-free synthesis and characteristics of acid–base bifunctionalized MCM-41 mesoporous silicas

A series of acid?Cbase bifunctionalized mesoporous silicas have been successfully synthesized by means of a convenient solvent-free approach. Through grinding the precursors, aluminium and magnesium nitrates, with the as-prepared MCM-41, and then the generation of acid?Cbase sites and removal of host template were simultaneously completed in the subsequent calcination procedure. The resultant modified mesoporous silicas MgO?CAl₂O₃?CMCM-41 were characterized with X-ray diffraction, high-resolution transmission electron microscopes, N₂ adsorption, FT-IR spectra, ²⁹Si and ²⁷Al MAS NMR, NH₃? and CO₂? temperature programmed desorption. The results indicate that the products exhibit excellent acid?Cbase properties with well mesoporous structure, and the guests were well dispersed in the channel of MCM-41, which make it exhibit high activity for the synthesis of EMC.     

Synthesis and Characterization of Gallosilicate Molecular Sieve with the MCM-22 Framework Topology

Ga-MCM-22 with high gallium content has been synthesized with hydrothermal method and characterized by means of X-ray diffraction, SEM, TGA, ⁷¹Ga and ²⁹Si MAS NMR, BET surface area, and NH₃-TPD. Ga-MCM-22 was obtained as very homogeneous particles of about 5-7 m diameters and showed very similar morphologies to its Al analogue. The Na/Ga ratio of Ga-MCM-22 was much lower than 1.0, indicating that most of the negative framework charges should be compensated by protonated template, HMI. The results from ⁷¹Ga and ²⁹Si MAS NMR demonstrated that gallium was incorporated into the tetrahedral framework of Ga-MCM-22. Also, the ²⁹Si MAS NMR spectra showed that the as-synthesized Ga-MCM-22 has a different distribution of tetrahedral Si sites from its Al analogue. After calcination, however, the distribution of tetrahedral Si sites significantly changed, due to a large degallation from the framework of Ga-MCM-22. The NH₃-TPD showed that Ga-MCM-22 possesses Brönsted and Lewis acid sites.     

Preparation of sulfated alumina supported on mesoporous MCM-41 silica and its application in esterification

New types of mesoporous SA/MCM-41 solid acid catalysts were prepared by loading sulfated alumina (SA) on MCM-41. The prepared catalysts were characterized by XRD, IR, N₂ physisorption, elemental analysis, FT-IR of adsorbed pyridine and NH₃-TPD. The esterification of acetic acid with n-butanol and citric acid with n-butanol were used as model reactions to test the catalytic activities and reusability of the SA/MCM-41 solid acid catalysts. Compared with SA catalyst, SA/MCM-41 catalysts exhibited higher catalytic performances, which were attributed to their high BET surface area and large pore volume. Moreover, 20SA/MCM-41 solid acid catalyst showed excellent reusability in both esterifications.