MCM-41 ordered mesoporous molecular sieves synthesis and characterization

Abstract

The aim of this work was to study the hydrothermal synthesis of Si and SiAlMCM-41 performed under both autogenic pressure and refluxing conditions. XRD data showed that the MCM-41 phase may be formed by both processes and that the synthesized material in the presence of Al and/or under reflux presents the hexagonally arrangement of less ordered mesopores. However, as verified by XRD and physisorption data, the order was improved with higher synthesis times. 29 Si and 1 H-29 Si C/P MAS NMR spectra showed that a great part of the Si atoms exists as silanol groups which originate resonance peaks at-110,-100 and-91 ppm. The presence of Al atoms may generate Si(3Si, Al) and Si(2Si, 2Al) environments which might be contributing to resonance peaks at-100 and-91 ppm. The 27 Al MAS NMR spectrum of the as synthesized AlSiMCM-41 showed a resonance peak of tetrahedral framework aluminum close to 53 ppm and two others, one close to 14 ppm attributed to Al(H2O)6 +3 species and the other a weak signal close to 32 ppm attributed to pentacoordinated Al. 27 Al MAS NMR spectra of the calcined sample showed a peak at 0 ppm corresponding to an hexacoordinated extra-framework aluminum formed during calcination.

Influence of Aluminum Addition in the Framework of MCM-41 Mesoporous Molecular Sieve Synthesized by Non-Hydrothermal Method in an Alkali-Free System

“…Furthermore, it was also observed a shoulder at -110 ppm which is found only on purely siliceous structures, being attributed to Q4 structural units, Si[(OSi) 4 ] 26,27 . Similarly, the 29 Si MAS-NMR spectra of solids Al-MCM-41(50) and Al-MCM-41 (15) showed two peaks, one intense at -99 ppm due to the presence of Si species, finding in their neighborhoods an aluminum atom, Si[(OSi) 3 (OAl)], and other with low intensity at -90 ppm due to the Si species with two or three aluminum neighboring atoms, Si[(OSi) 2 (OAl) 2 ] and Si[(OSi)(OAl) 3 ], respectively 18,22,[28][29][30] . In these last two 29 Si MAS-NMR resonance profiles, the corresponding peak at -110 ppm was decreasing with the increase the concentration of Al, indicating that aluminosilicates (Al-MCM-41) were formed.…”

“…On the other hand, the broadening of the peaks at 53 ppm suggests the presence of Al species in highly distorted tetrahedral structures 32 . Furthermore, it is believed when the material is subjected to heat treatment to remove the template surfactant (CTAB), a part of aluminum incorporated into the network coordination site undergoes a change probably caused by the breaking of Si-O-Al bonds, producing species of Al extra-framework or amorphous alumina occluded in the pores, making it a material with a low uniformity degree, without significantly changing the pore structure of the molecular sieve 30 . Nevertheless, the high intensity of the peaks 27 Al MAS-NMR at 53 and 0 ppm is also an indicative that the aluminum is present in the samples, mostly forming four-coordinated Al species, and in a less extent, forming six-coordinated Al species, which means that the aluminum atoms were actually incorporated into the silica network by non-hydrothermal methods at room temperature as achieved through direct hydrothermal synthesis 5 .…”

Properties and potential application of modern adsorbents in formulation of solid drug delivery systems

“…Prirodni porozni adsorbenti se dobijaju prečišćavanjem uzoraka iz prirode, koji sadrže određeni udeo karakterističnog adsorbenta, od standardnih nečistoća kao što su glina, vulkansko staklo, organske materije, šljunak, voda itd. Prednosti prirodnih adsorbenata u odnosu na sintetske su relativno jeftinija proizvodnja, koja uključuje samo proces prečišćavanja, i izostanak upotrebe toksičnih pomoćnih supstanci (12). Jedan od najnovijih prirodnih poroznih adsorbenata su dijatomitne silika mikrokapsule ili dijatomiti (13).…”

“…Dobijaju se prečišćavanjem dijatomejske zemlje. U literaturi se navodi veći broj postupaka za dobijanje dijatomita (12)(13)(14). Jedan od najčešće korišćenih podrazumeva prečišćavanje sirovog materijala sledećim procesima: usitnjavanje, uklanjanje velikih agregata dijatomita, uklanjanje oksidiranih nečistoća tretiranjem uzorka kiselinom, razdvajanje čestica po veličini pomoću filtracione ili sedimentacione metode (14).…”

Dialkylation of Naphthalene with Isopropanol Over H3PO4/MCM-41 Catalysts for the Environmentally Friendly Synthesis of 2,6-Dialkylnaphthalene

“…However, this change did not produce any structural distortions of the AlMCM-41. Consequently, it seemed reasonable to propose that loading of H 3 PO 4 on AlMCM-41 did not change the regular arrangement of uniform channels of the support [23]. It might be interesting to mention that by loading of approximately 30 wt% of phosphotungstic acid on AlMCM-41, the specific surface area of the support decreased by less than 20% [24], but loading the same support by approximately the same weight percent of H 3 PO 4 , the surface area decreased to a large extent.…”