Efficient catalysis of mesoporous Al-MCM-41 for Mukaiyama aldol reactions

Mesoporous aluminosilicates, Al-MCM-41 (Si/Al = 20 and 50), efficiently catalyzed Mukaiyama aldol reaction of benzaldehyde with 1-(trimethylsiloxy)cyclohexene in CH₂Cl₂ at 0 °C to afford the corresponding β-trimethylsiloxy ketone in quantitative yield. On the other hand, mesoporous silica (MCM-41), amorphous SiO₂–Al₂O₃, and H–Y and H-ZSM-5 zeolites barely catalyzed the reaction. Additionally, the less ordered Al-MCM-41 prepared by mechanical compression exhibited much lower catalytic activity compared with Al-MCM-41, indicating that the presence of the ordered mesoporous structure in aluminosilicates is crucial for the catalysis. The Al-MCM-41 catalyzed Mukaiyama aldol reaction was applicable to a wide range of aldehydes and silyl enol ethers. Furthermore, the Al-MCM-41 catalyst could be recycled at least three times without any loss in the yield. Thus, mesoporous aluminosilicates are promising heterogeneous catalysts for fine chemicals synthesis.     

The synthesis of Al-MCM-41 from volclay — A low-cost Al and Si source

The alkaline fusion of volclay (a low-cost sodium exchanged smectite) was used as source to generate the Si and Al components which were effectively transformed into mesoporous Al-MCM-41 depending on hydrothermal condition. The Al-MCM-41 materials were investigated by powder X-ray diffraction (XRD), N₂ adsorption–desorption measurements and both scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM). The volclay which converted into a silicon and an aluminium source allowed the formation of well ordered mesoporous Al-MCM-41 materials with high aluminium content (roughly 4 times higher than a Al-MCM-41 produced by a standard method), a high specific surface area (1060 m²/g), a pore volume of 0.8 cm³/g (for pore width < 7.1 nm) with an mono-modal pore distribution with a maximum in the mesoporous pore size of 3.8 nm in pore width.     

Design and synthesis of near-IR luminescent mesoporous materials covalently linked with tris(8-hydroxyquinolinate)lanthanide(III) complexes

By using the bifunctional ligand, 8-hydroxyquinoline-functionalized organosilane (Q-Si), the new mesoporous material Q–MCM-41 covalently bonded with 8-hydroxyquinoline was synthesized. Through the ligand exchange reaction, the new near-infrared (NIR) luminescent mesoporous LnQ₃–MCM-41 (Ln = Er, Nd, Yb) materials were prepared by linking the lanthanide quinolinate complexes to the ordered mesoporous Q–MCM-41 material. The LnQ₃–MCM-41 materials were characterized by powder X-ray diffraction and N₂ adsorption/desorption, and they all show the characteristic mesoporous structure of MCM-41 with highly uniform pore size distributions. Fluorescence spectra of these LnQ₃–MCM-41 materials were recorded and the corresponding luminescence decay analyses were measured. After ligand-mediated excitation, all the emission spectra of the LnQ₃–MCM-41 materials show the characteristic NIR-luminescence of the corresponding lanthanide ions via the intramolecular energy transfer from the ligands to the lanthanide ions. The good luminescent performances enable these NIR-luminescent mesoporous materials to have potential applications in optical amplification (operating at 1.3 or 1.5 μm) and laser systems, etc.     

Novel synthesis of ordered mesoporous materials Al-MCM-41 from bentonite

This paper reports the synthesis of ordered mesoporous materials Al-MCM-41 with a specific surface area of 1018 m²/g from bentonite. Pretreated bentonite was simultaneously used as silica and aluminum sources without addition of silica or aluminum reagents. Orthogonal experiments were adopted to optimize the processing parameters. The samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), N₂ adsorption–desorption measurements and Fourier transform infrared spectra (FTIR) techniques. The obtained materials were hexagonal Al-MCM-41. Calcination removed the surfactant while new bonds increased the crosslinking of the frameworks. Proper Si/Al molar ratio was critical for the formation of highly ordered mesoporous materials.     

Electrochemistry and determination of epinephrine using a mesoporous Al-incorporated SiO2 modified electrode

The potential application of Al-incorporated mesoporous SiO₂ (denoted as Al-MCM-41) in electrochemistry as a novel electrode material was investigated. The peak currents of K₃[Fe(CN)₆] remarkably increase and the peak potential separation obviously decreases at the mesoporous Al-MCM-41 modified carbon paste electrode (CPE). These phenomena suggest that the mesoporous Al-MCM-41 modified CPE possesses larger electrode area and electron transfer rate constant. Furthermore, the electrochemical behavior of epinephrine (EP) was investigated in different supporting electrolytes such as 0.01 mol L⁻¹ HClO₄ and pH 7.0 phosphate buffer. It is found that the mesoporous Al-MCM-41 modified CPE exhibits catalytic ability to the oxidation of EP due to remarkable peak current enhancement and negative shift of peak potential. The electrochemical oxidation mechanism was also discussed. Finally, a novel electrochemical method was proposed for the determination of EP, which used to determine EP in urine samples.     

Uptake of decontaminating agent from aqueous solution: a study on adsorption behaviour of oxalic acid over Al-MCM-41 adsorbents

Hydrothermal method was followed to synthesis the mesoporous Al-MCM-41 (Si/Al = 25, 50, 75 and 100) and Si-MCM-41 molecular sieves using a cetyltrimethylammonium bromide as a surfactant and the materials were unambiguously characterized by XRD, N₂ sorption studies, ²⁷Al MAS-NMR and TEM. The removal of oxalic acid from aqueous solution was studied through an adsorption process because oxalic acid may cause complexes with radioactive cations during decontamination operation in nuclear industry, which resulting in interferences in their removal by conventional treatment. Adsorption of oxalic acid over Al-MCM-41 shows the applicability of Langmuir isotherm and follows first order kinetics. The effects of parameters such as contact time, concentration of oxalic acid, adsorbents (various Si/Al ratios of Al-MCM-41, Si-MCM-41 and activated charcoal) and pH have been investigated to yield higher removal of oxalic acid. The percentage of oxalic acid adsorbed per unit gram of adsorbent for Al-MCM-41 at Si/Al = 100, 75, 50 and 25, Si-MCM-41, and activated charcoal are 34.6, 40.9, 51.4, 61.3, 16.1 and 60, respectively. Retainment of crystallinity and absence of structural collapse of Al-MCM-41 after desorption and adsorption of oxalic acid, respectively has been achieved in this study.     

Synthesis of highly stable mesoporous aluminosilicates from commercially available zeolites and their application to the pyrolysis of woody biomass

Ordered mesoporous aluminosilicates have been successfully obtained via the simple combination of top–down and bottom–up approaches, using commercially available zeolites as the framework sources. The mesoporous aluminosilicates are characterized by XRD, N₂ sorption, SEM, TEM, ICP, and NMR, and have proven to have controllable aluminium contents, well-developed mesoporosity, and excellent hydrothermal stability. The hydrothermally stable mesoporous aluminosilicates, which possess the proper distribution of weak and strong acid sites, are applied as reusable heterogeneous catalysts for the pyrolysis of woody biomass. The mesoporous aluminosilicates in the present work showed good activity, selectivity, and even stability for the production of desirable organic compounds such as phenolics, in comparison to conventional HZSM-5 and Al-MCM-41.     

Unique vapor phase synthesis of 1,2,3,5,6,7-hexahydrodicyclopenta[b,e]pyridine selectively over Co–Al-MCM-41

Cyclization of cyclopentanone, formaldehyde and ammonia in vapor phase gives 1,2,3,5,6,7-hexahydrodicyclopenta[b,e]pyridine (HHDCP) and spiro[cyclopentane-1,8′-(1′,2′,3′,5′,6′,7′,8′,8′a) octahydrodicyclopenta[b,e]]pyridine (SCOHDCP) over zeolites HY, HZSM-5, Hβ and mesoporous Al-MCM-41 molecular sieves. The preliminary screening of catalysts clearly shows that Al-MCM-41 is more suitable for the vapor phase synthesis of HHDCP. As the NH₃-TPD profiles of Al-MCM-41 show wide range distribution of acid sites in the temperature range of 200–600 °C (weak–medium–strong), Al-MCM-41 is further modified with transition metal ions like V(V), Mn(II), Fe(III), Co(III), Cu(II), La(III) and Ce(III) to fine tune the acid sites. Correlation of activity and selectivity of transition metal modified Al-MCM-41 with the NH₃-TPD profiles show that though the conversions are high, selectivity of either HHDCP or SCOHDCP is a preference of acid site strength formed on metal ion modification. Interestingly Co²⁺ ion modification of Al-MCM-41 resulted distinctly into two sets of acid sites with T_max around 218 °C (weak–medium) and 673 °C (strong). The reaction is studied on Co–Al-MCM-41 by adsorbing pyridine at 300 °C. The typical acidity available on pyridine adsorbed Co–Al-MCM-41 around 300 °C is showing cyclization activity forming only HHDCP indicating that weak–medium acid sites are responsible for the formation of HHDCP. Based on the product distribution plausible reaction mechanism is proposed.     

t-Butylation of p-cresol with t-butyl alcohol over mesoporous Al-MCM-41 molecular sieves

Selective liquid-phase t-butylation of p-cresol with t-butyl alcohol (t-BuOH) to produce 2-t-butyl-p-cresol (TBC) has been conducted over Al-MCM-41 catalysts with different Si/Al ratios. The effects of various reaction parameters such as temperature, reaction time and n_t-BuOH:n_p-cresol ratio on the conversion of p-cresol and the selectivity of TBC have been systematically investigated as well. When the Si/Al ratio of Al-MCM-41 catalysts is increased from 21 to 104 (respectively yielding Al-MCM-41(21), Al-MCM-41(42), Al-MCM-41(62), Al-MCM-41(83) and Al-MCM-41(104)), both the conversion of p-cresol and the yield and selectivity of TBC decrease due to the decrease of the number of Brønsted acid sites of the Al-MCM-41 catalysts. Al-MCM-41(21) catalyst is found to give the highest conversion of p-cresol (88.2%) and the highest selectivity of TBC (90.40%) under the optimal n_t-BuOH:n_p-cresol mole ratio of 2:1, the optimal reaction temperature of 90 °C and the optimal reaction time of 2 h. Furthermore, Al-MCM-41(21) can be recycled up to at least four times without losing its catalytic performance for butylation reaction.     

Activation of ethyl acetate over metal substituted MCM-41: application to alkylation and acylation

Al-MCM-41, Fe,Al-MCM-41 and Zn,Al-MCM-41 materials with different silicon to metal ratios were synthesized hydrothermally and characterized by XRD, BET, FT-IR, Acidity measurement by pyridine adsorbed FT-IR spectroscopy, ²⁹Si and ²⁷Al MAS NMR and ESR techniques. The orderly arrangement of mesoporous materials was clearly revealed from the XRD patterns. ²⁹Si and ²⁷Al MAS NMR established the co-ordination environment of silicon and aluminium. Electron paramagnetic resonance (EPR) study confirmed the co-ordination environment of Fe in Fe,Al-MCM-41 framework. The catalytic activity of these materials was evaluated in the vapour phase alkylation and acylation of ethylbenzene with ethyl acetate in the temperature range between 250 and 400 °C. The products were found to be 1,3-diethylbenzene (1,3-DEB), 1,4-diethylbenzene (1,4-DEB), 1,2-diethylbenzene (1,2-DEB), 4-ethylacetophenone (4-EAP) and acetophenone (AP). The reaction products revealed that activation of ethyl acetate is a convenient route for both alkylation and acylation reactions. The order of the catalysts activity for the reaction is found to be Fe,Al-MCM-41 (50) > Fe,Al-MCM-41 (100) > Zn,Al-MCM-41 (50) > Zn,Al-MCM-41 (100) > Al-MCM-41 (50) > Al-MCM-41 (100). In addition to the density of acid sites, the strength of acid sites is also important for this reaction. The effects of temperature, feed ratio, WHSV and time on stream were also examined and the results are discussed.     

Use of different mesostructured materials based on silica as cobalt supports for the Fischer–Tropsch synthesis

To obtain a novel, active and selective to diesel catalytic material for syngas processing via Fischer–Tropsch synthesis (FTS), a series of 20 wt.% cobalt catalysts has been prepared by impregnation of a mesoporous molecular sieve based on silica (SBA-15, Al-MCM-41, INT-MM1), and a commercial amorphous silica for comparison purposes. All materials were characterized by several physico-chemical techniques: AAS, BET surface area, XRD, TPR, and H₂ chemisorption with pulse reoxidation and finally their reactivity on the FTS reaction was evaluated at 523 K, 10 bar, and H₂/CO = 2. Catalytic and characterization results show a great influence of mesoporous support porosity on the structure, reducibility, and FTS catalytic behavior of cobalt oxide species supported over these ordered materials. It was found that the size of supported cobalt oxide species formed during the calcination step increased with the average pore size (D_p) of the mesoporous support. Thus, the catalyst with larger Co oxide species located in wide pore silica showed to be easily reducible, more active and very selective toward the diesel fraction. It seems to be the case of the Co/SBA-15 solid, which showed to be the most active solid (XCO 63%) when the same mass of catalyst was used. Under CO iso-conversion conditions (XCO 40%), Co/SBA-15 was more selective toward the formation of C₅₊ hydrocarbons (80%, α = 0.76) and less selective to CH₄ (15%). On the contrary, when Al-MCM-41 and INT-MM1 were used as supports, a lower selectivity to C₅₊ and CO conversion and higher CH₄ selectivity (20%) were observed due to the decrease of D_p, of the cobalt oxide species size and the reducibility degree of such species.     

Synthesis of Ti-containing mesoporous silicates from inorganic titanium sources

Titanium-containing mesoporous molecular sieves including periodic mesoporous silicas (SBA-15-type) and organosilicas (PMO-type) can be assembled by using mixed inorganic acid–base pairs (TiCl₄ and tetrabutyl titanate) or a single inorganic TiCl₃ as the titanium sources and tetraethoxysilane and/or 1,2-bis(triethoxysilyl)ethane as the silica sources and triblock copolymer as the structure-directing agent in acidic media through the hydrothermal method. Characterization using XRD, nitrogen sorption isotherms, UV–vis, FT-IR and NMR techniques reveals that the Ti-containing mesoporous materials possess ordered 2D hexagonal mesostructures, high surface areas (421–1070 m²/g), uniform pore sizes (5.1–8.0 nm), large pore volumes (0.5–1.3 cm³/g), and tetrahedrally incorporated titanium (IV) species in the silica network. The maximum incorporated Ti content is about 0.34 wt% for the ordered mesostructure regardless of the titania and silica sources and the initial Si/Ti ratio.     

A simple co-impregnation route to load highly dispersed Fe(III) centers into the pore structure of SBA-15 and the extraordinarily high catalytic performance

Highly dispersed iron centers supported on SBA-15 were successfully prepared via a simple incipient wetness co-impregnation route by casting furfuryl alcohol (FA) solution of iron (III) acetylacetonate (Fe(acac)₃), which were used as carbon and iron sources, respectively, into the pore structure of SBA-15, followed by the subsequent removal of carbonized FA. Various techniques such as XRD, TEM, N₂ sorption, UV–vis, XPS and EPR, were employed to characterize the prepared catalysts. It was shown that both Fe₂O₃ nanoclusters and isolated iron species were present and highly dispersed onto the pore surface of SBA-15, due to the presence of abundant carbon source co-impregnated, with well-maintained, highly ordered and open mesoporous structure. A great number of acidities was introduced by the loading of Fe₂O₃, and the catalytic performance was tested on the Friedel–Crafts benzylation of benzene by benzyl chloride. Under the optimized reaction condition, the catalyst showed a superior catalytic performance with a 100% yield of monoalkylated product within 1.5 min at 60 °C. The catalyst demonstrated high reusability and stability, the yield of diphenylmethane was still higher than 90% after 6 runs. Moreover, the catalyst was still active at the temperature as low as 40 °C. Such a strategy is verified applicable to prepare other well-dispersed metal oxides, i.e. Mn_xO_y loaded into the pore structure of mesoporous materials.     

Hybrid zeolitic-mesostructured materials as supports of metallocene polymerization catalysts

The potential application of hybrid ZSM-5/Al-MCM-41 zeolitic-mesostructured materials as supports of metallocene polymerization catalysts has been investigated and compared with the behaviour of standard mesoporous Al-MCM-41 and microporous ZSM-5 samples. Hybrid zeolitic-mesostructured solids were prepared from zeolite seeds obtained with different Si/Al molar ratios (15, 30 and 60), which were assembled around cetyltrimethylammonium bromide (CTAB) micelles to obtain hybrid materials having a combination of both zeolitic and mesostructured features. (nBuCp)₂ZrCl₂/MAO catalytic system was impregnated onto the above mentioned solid supports and tested in ethylene polymerization at 70 °C and 5 bar of ethylene pressure. Supports and heterogeneous catalysts were characterized by X-ray powder diffraction, nitrogen adsorption-desorption isotherms at 77 K, transmission electron microscopy, ²⁷Al-MAS-NMR, ICP-atomic emission spectroscopy and UV-vis spectroscopy.Catalysts supported over hybrid ZSM-5/Al-MCM-41 (Si/Al = 30-60) exhibited the best catalytic activity followed by those supported on Al-MCM-41 (Si/Al = 30-60). However, catalyst supported on ZSM-5 gave lower polymerization activity because of its microporous structure with narrower pores and lower textural properties than hybrid and mesoporous materials.Although higher acid site population shown by hybrid materials could contribute to the stabilization of the metallocene system on the support, in this case their better catalytic performance is mainly ascribed to the larger textural properties.     

Synthesis, characterization, and catalytic properties of a hydrothermally stable Beta/MCM-41 composite from well-crystallized zeolite Beta

A Beta/MCM-41 composite has been synthesized with a new method by using well-crystallized zeolite Beta as silica and aluminum source. The prepared composite was characterized by XRD, FTIR, N₂ adsorption/desorption at 77 K, FE-SEM, DTG, ²⁹Si MAS NMR spectral techniques. It was shown that the composite consisted of a highly ordered mesoporous MCM-41 phase and a zeolite Beta phase. Its hexagonal mesoporous structure was still retained after statically treated for 120 h in boiling water. In contrast, the structure of generally synthesized Al-MCM-41 nearly completely collapsed. This might be attributed to the assembly of the dissolved fragments such as the first and/or secondary structural units of zeolite Beta into the mesoporous structure around surfactant micelles. This is supported by the catalytic result that the prepared composite showed higher activity and selectivity for medium fraction in hydrocracking of Daqing vacuum residue than the parent zeolite Beta, the Al-MCM-41 as well as the mechanical mixture of these two materials.     

Catalytic performance of metal ion doped MCM-41 for methanol dehydration to dimethyl ether

Metal ion doped MCM-41 mesoporous molecular sieves (M-MCM-41, M = Al, Ga, Sn, Zr and Fe) were prepared using a hydrothermal synthesis method, with metal chlorides serving as the dopant sources. The M-MCM-41 structures were characterized by Fourier transform infrared spectroscopic (FTIR) analysis, X-ray diffraction, energy dispersive spectroscopy and N₂ adsorption–desorption measurement. The surface of M-MCM-41 acidities were determined by NH₃ temperature-programmed desorption and pyridine-adsorption FTIR analysis, and their catalytic performance for methanol dehydration to dimethyl ether (DME) was evaluated. The results showed that the prepared M-MCM-41, which exhibited a structure similar to that of MCM-41 with long-range ordered mesoporous structure, contained weak acidic sites. The number of weak acid sites in Al-MCM-41 increased as the Al content increased. The Al content in Al-MCM-41 had an important effect on its catalytic performance, where the highest catalytic activity was 80 and 100 % DME selectivity was achieved at a Si/Al molar ratio of 10. For MCM-41 doped with various types of metal ions, M-MCM-41 (M = Al, Ga, Sn and Zr) also presented a similar wide distribution of acidity, and their catalytic activities were ranked in the following order: Al-MCM-41 > Ga-MCM-41 > Zr-MCM-41 > Fe-MCM-41 > Sn-MCM-41, which were related to the coordination of the metal ions.     

An investigation of the performance of catalytic aerogel filters

Gas permeable, photoactive and crack-free titania–silica aerogels of high titanium content (i.e., up to Ti/Si = 1) were prepared by two-steps acid–base catalyzed method involving an acid-catalyzed prehydrolysis of silicon alkoxide followed by a base-catalyzed hydrolysis/condensation reactions with a chelated titania precursor. The prepared titania–silica aerogels displayed good mechanical strength (>30 kN m⁻²), large surface area (>550 m²/g), mesoporous structure (8–11 nm) and good gas permeation. The porous aerogels trap and filter airborne particulates and the titania–silica aerogel have a fair performance for aerosol (65%) and bioaerosol (94%) filtrations. The photoactive anatase nano-TiO₂ crystallized within the aerogel displays an order of magnitude higher reaction rate for UVA photooxidation of trichloroethylene compared to commercial Degussa P25 TiO₂. The bactericidal activity of the titania–silica aerogel for Bacillus subtilis cells under UVA was also six orders of magnitude better.     

Development of mesoporous Al,B-MCM-41 materials: Effect of reaction temperature on the catalytic performance of Al,B-MCM-41 materials for the cyclohexanone oxime rearrangement

A series of aluminum–boron–silicate MCM-41 mesoporous materials and their counterparts treated with NH₄F aqueous solution were synthesized and characterized by using XRD, MAS NMR, nitrogen physisorption, DRIFT, TG-DTA, TP/MS and pyridine adsorption. All of the samples showed typical MCM-41 structural and textural properties. ²⁷Al MAS NMR showed that the aluminum environment was mainly four-coordinated and six-coordinated aluminum for non-fluorinated samples and fluorinated ones, respectively. Boron was in the trigonal framework environment at ca. catalytic reaction temperatures and the NH₄F treatment did not affect the boron environment in our Al,B-MCM-41 materials. All of the Al,B-MCM-41 materials studied contained both Brønsted and Lewis acid sites. However, the strong acid Brønsted/Lewis ratios decreased in the fluorinated catalysts. Moreover, the influence of temperature was studied on the cyclohexanone oxime conversion and the product selectivity in the 623–798 K range. Results indicated that temperatures lower than 748 K favored Beckmann rearrangement to -caprolactam, whereas, at higher temperatures the main reaction was cyclohexanone oxime hydrolysis to cyclohexanone. The aluminum–boron–silicate MCM-41 mesoporous materials treated with NH₄F improved both the selectivity to -caprolactam (related mainly to boron content) and their life span (related to their lower ratios of strong Brønsted/Lewis acid sites).     

Cobalt oxide nanoparticles on mesoporous MCM-41 and Al-MCM-41 by supercritical CO2 deposition

CoO and Co₃O₄ nanoparticles were uniformly dispersed inside mesoporous MCM-41 and Al-MCM-41 supports using supercritical CO₂ reactive deposition. This method represents a one-pot reproducible procedure that allows the dissolution of the organocobalt precursor and supports impregnation in supercritical CO₂ at 70 °C and 110 bar, followed by the precursor thermal decomposition into cobalt species at 200 °C and 160 bar. By the relative concentration of the cobalt precursor [cobalt (II) bis (η⁵-ciclopentadienil)], the load of cobalt nanoparticles was controlled and then determined by Inductively Coupled Plasma (ICP-OES). The synthesis of CoO and Co₃O₄ species inside the MCM-41 and Al-MCM-41 substrates was confirmed by X-ray Photoelectron (XPS) and Laser Raman Spectroscopies (LRS). By N₂ adsorption and Small Angle X-ray Scattering (SAXS), it was determined that the hexagonal arrangement as well as the surface area and pore size of the substrates changed after the addition of cobalt. By means of X-ray mapping from SEM images, a homogeneous distribution of cobalt nanoparticles was observed inside the mesopores when the cobalt loading was 1 wt.%. In addition, spherical cobalt nanoparticles of average diameter close to 20 nm were detected on the outer surface of MCM-41 and Al-MCM-41 supports when the cobalt content was higher. On the other hand, by Transmission Electron Microscopy (TEM), it was possible to measure the interplanar distance of the crystalline plane of the outer nanoparticles, which was later compared with the theoretical distance values which allowed identifying the CoO and Co₃O₄ phases.