Acidic SiOH Research Articles

Mechanistic insight into the improved photocatalytic degradation of dyes for an ultrathin coating of SiO2 on TiO2 (P25) nanoparticles

The photocatalytic mechanism of TiO2 (P25) nanoparticles coated with SiO2 (SiO2:TiO2) by atomic layer deposition was investigated. The deposition of SiO2 on TiO2 not only gives a photocatalytic improvement for the degradation of both Rhodamine B (3.6–fold) and Acid Blue 9 (3–fold). SiO2 deposition also changes the mechanism from direct oxidation of the pollutant at the surface of TiO2 to a predominantly OH⁎ radical based degradation of the pollutants originating from SiO2:TiO2. Low SiO2 loadings on TiO2, where the coating is incomplete, improve the OH⁎ radicals generation due to the higher number of acidic Si–OH groups combined with the facilitated charge separation at the TiO2–SiO2 interface. As a consequence of incomplete coverage, the TiO2 surface remains accessible, which allows both the oxidation and reduction reactions at the SiO2:TiO2 surface. On the other hand, high loading of SiO2 (>3 wt.% Si) results in photocatalytic suppression due to the coverage of TiO2 surface by SiO2. The degradation of differently charged dyes on the SiO2:TiO2 surface demonstrates the independence of the adsorption properties on the photocatalytic improvement. Simultaneous degradation of two dyes demonstrated the advantage of SiO2:TiO2 being less selective and, therefore, better suited for general water purification.

Acidity enhanced [Al]MCM-41 via ultrasonic irradiation for the Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam

Using solid acid catalysts to replace liquid acids in the liquid-phase Beckmann rearrangement of cyclohexanone oxime (CHO) into ɛ-caprolactam (CPL) is crucial for the environmentally friendly production of synthetic fibers, such as Nylon-6. In this work, we prepared aluminum-containing MCM-41 catalysts under ultrasonic irradiation with various Si/Al ratios for this purpose. Quantitative 1H MAS NMR investigations show that ultrasonic irradiation significantly promotes the formation of active Brønsted acid sites (BAS) on the [Al]MCM-41 catalysts up to 8 times higher than those prepared at the same conditions without ultrasonic irradiation, and up to 12 times higher BAS density than those reported in the literatures. The catalytic performance of [Al]MCM-41 catalysts can be strongly improved with increasing the BAS density, particularly to the ratio of BAS/(weakly acidic SiOH groups). Moreover, [Al]MCM-41 catalysts dehydrated at 393 K obtained two time higher CHO conversion and CPL yield than that dehydrated at 473 K. Hydrogen-bonded water molecules retained at low dehydration temperature may block surface SiOH groups and promote the reaction process. With higher BAS density resulting from ultrasonic irradiation, [Al]MCM-41 catalyst (Si/Al = 10) in this work obtained the highest CPL yield among all [Al]MCM-41 materials reported for liquid-phase Beckmann rearrangement up to now. Finally, the reusability of [Al]MCM-41 catalyst was tested and no significant activity loss can be observed after five reaction cycles.

Mechanisms of the AlCl3 Modification of Siliceous Microporous and Mesoporous Catalysts Investigated by Multi-Nuclear Solid-State NMR

Solid-state NMR spectroscopy was utilized for investigating the post-synthetic formation of surface acid sites via AlCl3 modification of dealuminated zeolite Y (DeaY) and siliceous mesoporous SBA-15 and subsequent thermal treatment. Upon calcination at 723 K, aluminum atoms introduced by the AlCl3 modification coordinate at Q2 (Si(2Si,2OH)) and Q3 (Si(3Si,1OH)) sites and form tetrahedrally coordinated (AlIV) framework aluminum species. Most of the spectroscopically observed pentacoordinated (AlV) and octahedrally coordinated (AlVI) aluminum atoms are extra-framework species, partially acting as Lewis acid sites (LAC). The aluminum coordination at the framework is accompanied by the formation of Bronsted acidic SiOH groups with weak acid strength. The weak Bronsted acidity of these SiOH groups is explained by neighboring framework aluminum atoms with strongly disturbed tetrahedral oxygen coordination. For zeolite DeaY, in addition to the aluminum incorporation into silanol nests, also the aluminum incorporation via surface reactions of AlCl3 with the intact SiO2 framework occurs. In the case of mesoporous SBA-15, with a fivefold higher density of SiOH groups of the parent material compared with that of the parent zeolite DeaY, the aluminum incorporation into silanol nests is the dominating mechanism. By solid-state NMR spectroscopy of the dehydrated samples loaded with probe molecules, significantly larger densities (factor of 3–8) of LAC compared with those of Bronsted acid sites were determined for the AlCl3-modified and calcined zeolite DeaY and mesoporous SBA-15.

The influence of concentration on specific ion effects at the silica/water interface

Second harmonic generation spectroscopy is a useful tool for monitoring changes in interfacial potential at buried insulator/liquid interfaces. Here we apply this technique to the silica/aqueous interface and monitor the changes in interfacial potential while varying the pH in the presence of different alkali halides at 0.1M concentration. Within the pH range explored, the bimodal distribution of acidic sites on planar silica is clearly observed, corresponding to two types of acidic SiOH groups. Comparing these data with previous work at 0.5M sheds light on whether the presence of the ions stabilizes the charged or neutral state of the surface sites. For the alkali chlorides, with the exception of NaCl, we observe that the presence of the alkali chlorides stabilize the less acidic site in the protonated (SiOH) rather than deprotonated (SiO−) form. This unusual influence of the cation is attributed to the combination of interactions at the interface between water, surface sites and the electrolyte. Overall, we observe that the influence of the alkali ion on the ratio of the two types of sites and their effective acid dissociation constants is minor at 0.1M, unlike that observed at 0.5M. In contrast, the influence of the anion on the cooperative dissociation of surface sites and their relative distribution is little affected upon decreasing the concentration, which indicates that these specific anion effects are prevalent in nature.

Impact of Conventional and Microwave Heating on SAPO‐5 Formation and Brønsted Acidic Properties

AbstractThe hydrothermal crystallization of SAPO‐5 molecular sieves with Si mole ratios in the range of 0.05 to 0.13 was performed by either conventional or microwave heating to compare the influence of these two techniques on the isomorphous substitution of phosphorus by silicon atoms in the aluminophosphate framework. An extensive characterization of the obtained silicoaluminophosphates by ICP‐OES, SEM, XRD, TPDA, and MAS NMR spectroscopy indicated that conventional heating (drying oven) led to a better crystallinity and a higher content of isolated framework silicon atoms than microwave heating. While microwave heating has the advantage of reducing the crystallization time and conventionally prepared SAPO‐5 materials show better incorporation of isolated silicon atoms into the AFI framework according to the substitution mechanism SM IIa. This lower SM IIa substitution of tetrahedral framework sites by isolated silicon atoms in microwave‐prepared SAPO‐5 materials is the reason for their lower density of Brønsted acidic bridging OH groups [Si(OH)Al]. In contrast, the density of defect OH groups, such as weakly acidic SiOH groups, is higher than for conventionally prepared samples.

Zeolite- and MgO-supported rhodium complexes and rhodium clusters: Tuning catalytic properties to control carbon–carbon vs. carbon–hydrogen bond formation reactions of ethene in the presence of H2

Essentially molecular rhodium catalysts were made from Rh(C2H4)2(acetylacetonate) on zeolite HY and on MgO and characterized by infrared and X-ray absorption spectroscopies. The supported rhodium species anchored to the zeolite, initially in the form of Rh(C2H4)2, selectively catalyzed ethene dimerization, typically at 298K and 1bar, but when the catalyst was poisoned by CO, or the support was changed to MgO or zeolite NaY, or the rhodium was converted into small clusters, the ethene underwent predominantly hydrogenation. The preciseness of the synthesis of the supported rhodium species facilitated determination of structure-catalyst performance relationships that led to a schematic representation of how the dimerization proceeds by a mechanism involving both the rhodium complexes and zeolite surface OH groups. The reaction is facilitated by H2 and proceeds as one ethene molecule is activated by an isolated rhodium complex and another by a weakly acidic Si–OH–Al group.

Modes of Interaction of Simazine with the Surface of Model Amorphous Silicas in Water

Simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) was adsorbed from aqueous solutions on three types of amorphous silica, one commercial nonporous and two sol–gel with different porosity. Adsorption takes place on both types of solids in a narrow pH interval around 5.5. This fact, pH changes along adsorption, and IR spectroscopic data concerning dried samples provide evidence that adsorption involves proton transfer from acidic SiOH species to N atoms at ethylamino chains, those at the ring being practically not basic. Two adsorbate species are involved with markedly different interaction enthalpy, causing the presence of two regions in the pseudoisotherms, one irreversibly held, the other partially reversible. Possibilities for the occurrence of two species are discussed. The most likely explanation seems to be a two steps process, in which first a protonated molecule is formed, onto which a second molecule may anchor so forming a dimer, held together by either sharing of the positive charge or hydrophobic interactions. The available surface appears to have basically the same properties in all three cases. Microporosity, however, introduces diffusional constraints and limits the extent of surface available to adsorption.

Hierarchic zeolites: Zeolite ZSM-5 desilicated with NaOH and NaOH/tetrabutylamine hydroxide

The desilication of zeolite ZSM-5 (Si/Al=31.6) was performed by the treatment with NaOH of various concentrations (0.1, 0.2, 0.5 and 1M) and with NaOH/TBAOH mixtures of various concentrations and ratios between both bases. The information on the composition of both bulk and surface was obtained by ICP-OES and XPS, respectively. It has been found that diluted NaOH solutions extracted Si equally from the surface and bulk of ZSM-5. In more concentrated NaOH the desilication from the bulk was more effective than from the surface. The MAS NMR studies showed that Si(4Si,0Al) was less stable and was removed from the zeolite structure before Si(3Si,1Al). Higher stability of Si(3Si,1Al) was explained by the repulsion of AlO4- and OH−. According to the IR results the desilication, leading to mesopore formation, increased distinctly the amount of Si–OH on mesopore walls and also the contribution of acidic Si–OH–Al. Calcination of desilicated zeolite at air atmosphere resulted in a significant loss of tetrahedral Al due to the “steaming” of zeolite with water physisorbed in mesopores facilitated by lower stability of framework injured by desilication. Both NMR and IR studies suggested that the stability of Si atoms in the zeolite framework increased in the following order: (SiO)3Si–OH<(SiO)3Si–OSi<(SiO)3Si–OAl. Porosimetric experiments evidenced that the NaOH treatment produced mesopores with very broad pore size distribution (a majority of pore diameters was close to 20nm). The desilication with the NaOH/TBAOH mixtures produced mesopores of higher surface area and pore volume but of smaller diameter than with NaOH alone. We suppose that in the presence of TBAOH a larger number of narrower pores is formed, or narrow pores penetrate zeolite crystal more deeply.

Acidity of MCM-58 and MCM-68 zeolites in comparison with some other 12-ring zeolites

Two novel zeolites MCM-58 and MCM-68 with 12-ring channels and containing the cages in the structure were investigated with respect to their acidic properties and compared with other two 12-ring zeolites, namely Beta and ZSM-12. The main objective was to characterize the accessibility and acid strength of Lewis and Brønsted sites using FTIR spectroscopy. Pyridine adsorption allowed for quantification of Brønsted and Lewis acid centres and pointed out that while in the structure of MCM-58 nearly all Al atoms form acidic Si OH Al bridges, only about half of Al is a source of the Brønsted acidity in MCM-68. The special attention was devoted to the origin and location of the hydrogen-bonded OH groups in the MCM-58 zeolite. The frequency lowering is caused by the across-the-ring interaction with the oxygen atom and not by the interaction with the nearby OH groups, as proved by high temperature activation and deuteration experiments. The location of the 3485 cm −1 OH groups in the five member ring was proposed. Carbon monoxide adsorption allowed proposing the location of the residual K + cations in the main channels of MCM-58 and in the 6 104 6 cages in MCM-68.

Basic sites in zeolites followed by IR studies of NO+

Basic properties of zeolites of faujasite type in sodium and protonic forms as well as in zeolites steamed and dehydroxylated was investigated. NO + was used as a probe for basic sites. Three kinds of basic sites (framework oxygens) of various basicity were found in zeolite NaY. The basicity of framework oxygens was modified by the adsorption of electrondonor organic molecules. Spectra of increasing amount of NO + and of NO 3 − (spectra a–d) formed in zeolite NaY. The basic properties of zeolitic oxygens were examined by IR studies of NO + ions. NO + were formed by the disproportionation of N 2 O 4 (N 2 O 4 → NO + + NO 3 − ). NO + were stabilized by zeolitic oxygens and NO 3 − by the zeolitic cations or positively charged extraframework Al species. The spectra of NO + in zeolite NaY show three maxima that indicate the presence of oxygens of various negative charges (of various basicity). These oxygens may have various numbers of AlO 4 − and/or various numbers of Na + in close vicinity. The basic oxygen content was estimated as 15 sites/u.c. which is less than 5% of all the oxygen atoms in faujasite-type zeolite. In the presence of water, N 2 O 4 disproportionates producing: NO 3 − , NO 2 − and strongly acidic Si–OH–Al groups. Similarly, NO + reacts with water, forming NO 2 − and Si–OH–Al. The presence of strongly acidic Si–OH–Al decreases the basicity of the zeolitic oxygens. The basicity of the zeolitic oxygens diminishes also with the Si/Al, due to the decreased amount of AlO 4 − . NO + were also formed in zeolites NaHY but like NO 3 − , their amount decreased with the degree of Na/H exchange, i.e., with the amount of Na + cations, which are supposed to stabilize NO 3 − ions. In zeolites NaHY the basicity of oxygens decreased with the degree of Na/H exchange, due to replacement of Na + cations by more electronacceptor H + ions, but this effect is smaller than the variation of Si/Al The basicity of the oxygens in dehydroxylated zeolite NaHY was found to be comparable (but somewhat lower) to that in zeolite NaY. Steamed zeolite HY (which is known to contain the most strongly acidic hydroxyls) showed the lowest basicity of the zeolitic oxygens. IR studies showed also that the basicity of the zeolitic oxygens in zeolite NaY increased in the presence of electrondonor organic molecules such as CCl 4 and chloroform, which we relate to some “neutralization” of the zeolitic cations.

IR and NMR studies of mesoporous alumina and related aluminosilicates

The synthesis of mesoporous alumina and aluminosilicates of Si/Al from 0.2 to 10 was done, and the status of Si and Al was followed by 29Si and 27Al MAS NMR. The properties of acid sites were studied by following the IR spectra of CO, N 2, pyridine and ammonia adsorbed. The mesoporous alumina contained only surface weak Lewis acid sites, the insertion of Si resulted in loss on such Lewis sites and in the formation of Brønsted sites. The Al OH bands decreased (the Al IV OH band related to tetrahedral Al species diminished the first) and Si OH Al one (IR band at 3605 cm −1)—similar as in the case of zeolites appeared as well as Si OH groups (IR band at 3740 cm −1). The aluminosilicates of higher Si content contained also strong Lewis acid sites formed by the dehydroxylation of Si OH Al groups (process similar as in zeolites), which occurred at the activation. The concentration of both Lewis and Brønsted sites was determined by quantitative IR studies of ammonia and pyridine adsorption. The concentration of Brønsted acid sites increased with Si content due to the formation of acidic Si OH Al, and at high Si/Al, it increases also due to the growing surface area. The concentration of Lewis sites decreased with Si content due to the transformation of surface Al species, which are potential Lewis sites into Si OH Al and also to dilution of the material with Si species. The acid strength of Lewis sites increased with Si content due to increasing contribution of strongly acidic sites produced by dehydroxylation. The mesoporous aluminosilicate (Si/Al = 10) studied in this research showed higher concentration and higher acid strength of Brønsted sites than HMCM-41 of the same Si/Al—it may be an potential catalyst in fine chemistry reactions.

Selective synthesis of cumene and p-cymene over Al and Fe silicates with large and medium pore structures

The alkylation of benzene and toluene with isopropanol was studied in relation to time-on-stream (TOS) at temperatures of 520 and 570 K with the large pore zeolites Y, beta, mordenite and ZSM-12 and medium pore MFI silicates with isomorphously substituted Al and Fe in the framework. It has been found that the conversion of toluene and benzene does not correlate with the number and strength of bridging OH groups, as it does for alkylation reactions leading to xylenes and ethyltoluenes. The TOS behaviour of the conversion as well as the product composition indicated that the reaction rate is controlled by the desorption/transport rate of the bulky propylbenzenes and propyltoluenes being affected by the acidity and channel geometry of the zeolite. Formation of the undesired n-propylbenzene and n-propyltoluenes via bimolecular transalkylation reactions is enhanced by the acidity (strength and number of OH groups) and narrow intersecting channels of the molecular sieves. Thus, on acid catalysts with a more open structures (Y, beta, AlCl 3, H 3PO 4) the formation of branched isomers is preferred. However, as these zeolites do not exhibit a high para-selectivity needed for synthesis of p-cymene, ferrisilicates of MFI structure possessing less acidic SiOHFe groups are favoured.

Comprehensive characterization of some silica-based stationary phase for high-performance liquid chromatography

Comprehensive studies have been carried out on the effect of the silica support in preparing optimized bond-phase packing for high-performance liquid chromatography. Different covalently bonded reversed-phase silica-based chromatographic materials have been investigated with respect to lingand density, residual silanol group type, and concentration. Results from diffuse-reflectance Fourier-transform infrared spectroscopy, elemental and thermogravimetric analysis, and 1H solid-state- and 29Si cross-polarization magic-angle spinning nuclear magnetic resonance techniques were combined with liquid chromatographic data to provide quantative insights regarding the properties of silica-based bonded-phase packings. We have determined that unfavorable adsorption of basic compouds and low hysdrlytic stability of alkyl bonded-phase ligands can attrilbuted to the existence of siolated, non-hydrogen-bridged, high acidic SiOH groups. Contrary to comon belief, a stable silica support with the lowest adsorptivity for basic compounds must contain the highest possible (and not the lowest) concentration of homogeneously distributed, associated or bonded SiOH groups to ensure a minium concentration of deleterious highly acidic, isolated silanols. Under these conditions, silica-based bonded-phase packings exhibit higher hydrolytic staility and a substantially lower order of adsorptivity towards basic organic compounds.

New patent listing

The alkylation of benzene and toluene with isopropanol was studied in relation to time-on-stream (TOS) at temperatures of 520 and 570 K with the large pore zeolites Y, beta, mordenite and ZSM-12 and medium pore MFI silicates with isomorphously substituted Al and Fe in the framework. It has been found that the conversion of toluene and benzene does not correlate with the number and strength of bridging OH groups, as it does for alkylation reactions leading to xylenes and ethyltoluenes. The TOS behaviour of the conversion as well as the product composition indicated that the reaction rate is controlled by the desorption/transport rate of the bulky propylbenzenes and propyltoluenes being affected by the acidity and channel geometry of the zeolite. Formation of the undesired n-propylbenzene and n-propyltoluenes via bimolecular transalkylation reactions is enhanced by the acidity (strength and number of OH groups) and narrow intersecting channels of the molecular sieves. Thus, on acid catalysts with a more open structures (Y, beta, AlCl3, H3PO4) the formation of branched isomers is preferred. However, as these zeolites do not exhibit a high para-selectivity needed for synthesis of p-cymene, ferrisilicates of MFI structure possessing less acidic SiOHFe groups are favoured.