Kinds Of Acid Sites Research Articles

Surface Properties and Microstructures of Al-SBA-15 under Different Temperature Calcinations in Relation to Adsorption Performance

In this study, the calcination temperature was adjusted to influence the properties, including surface alumina types, microstructure, and surface acidity, and then to explore their association with the performance of As(V) adsorption after preparing aluminum-modified SBA-15 (Al-SBA-15x) by the incipient wetness impregnation method. The results showed that calcination promoted the formation of aluminum oxides with different coordination environments on SBA-15 carriers, resulting in different kinds of acid sites and Al–OH groups, which were the adsorption sites for As(V). However, with the increase in the calcination temperature, the total surface acidity on the Al-SBA-15x samples gradually decreased, and the dehydration and dehydroxylation reactions of aluminum oxides occurred, which reduced the Al–OH groups and changed the acid–base environment on the samples. Besides, high-temperature calcination caused a partial collapse of mesoporous channels, which was not conducive to dispersing the active adsorption centers on the surface and further reduced the effective utilization of active adsorption centers. Therefore, the adsorption capacity of an Al-SBA-15x adsorbent for As(V) decreased with the increase of the calcination temperature. Furthermore, kinetic models were used to investigate the relationship between the microstructure and adsorption processes. Finally, recycling tests were performed to demonstrate the reusability of Al-SBA-15x samples.

Enhancing catalytic epoxide ring-opening selectivity using surface-modified Ti3C2T x MXenes

MXenes are a new family of two-dimensional carbides and/or nitrides. Their 2D surfaces are typically terminated by O, OH and/or F atoms. Here we show that Ti3C2T x —the most studied compound of the MXene family—is a good acid catalyst, thanks to the surface acid functionalities. We demonstrate this by applying Ti3C2T x in the epoxide ring-opening reaction of styrene oxide (SO) and its isomerization in the liquid phase. Modifying the MXene surface changes the catalytic activity and selectivity. By oxidizing the surface, we succeeded in controlling the type and number of acid sites and thereby improving the yield of the mono-alkylated product to >80%. Characterisation studies show that a thin oxide layer, which forms directly on the Ti3C2T x surface, is essential for catalysing the SO ring-opening. We hypothesize that two kinds of acid sites are responsible for this catalysis: In the MXene, strong acid sites (both Lewis and Brønsted) catalyse both the ring-opening and the isomerization reactions, while in the Mxene–TiO2 composite weaker acid sites catalyse only the ring-opening reaction, increasing the selectivity to the mono-alkylated product.

Role of organic/inorganic-sulfur doping on photocatalytic behavior of mesoporous bimetallic oxides heterojunction

Abstract A series of novel sulfur-doped mesoporous bimetallic oxides with controllable structure and property were synthesized for acid-induced photo-Fenton. The one-pot sol-gel strategy allowed high-dispersion and strong adhesion of different organic-/inorganic-sulfur species (including CH4N2S, C5H11NO2S, (NH4)2SO4 and Na2S2O3) in host matrix to achieve highly active bimetallic heterojunction with stronger acid strength. It was found that organic-sulfur doping tended to simultaneously produce Bronsted and Lewis acid while inorganic-sulfur doping only generated Lewis acid. When two kinds of acid sites appeared on surface of solid, it made significant contributions toward superior performance either in absence of light (90.7% decoloration) or at pH of 10.0 (97.7% decoloration and 60.9% TOC removal). Possible sulfuration mechanism of S-doped bimetallic oxides for H2O2 activation had been explored in details. The acidity of solids was inversely related to chain length of organic-sulfur but positively correlated with electronegativity of inorganic-sulfur. The study will not only make a facile and effective way for developing highly-efficient heterogeneous catalysts, but also provide valuable insights for understanding relationship between surface physicochemical properties and catalytic activity.

2D, 3D mesostructured silicas templated mesoporous manganese dioxide for selective catalytic reduction of NOx with NH3

Employing nanocasting method, mesoporous MnO2 catalysts with large specific surface area and regular pore structure were synthesized by using three different types of mesostructure silicas (KIT-6, SBA-15, MCM-41) as hard templates. The physical and chemical properties of the three different mesostructured manganese oxide materials were comparatively and systematically characterized by using XRD, FT-IR, N2 adsorption-desorption, TEM, NH3-TPD, H2-TPR, XPS. The NH3-SCR performance of the mesoporous MnO2 was also evaluated on SCR self-assembly device. It was discovered that the mesostructure of the template would influence the amount and the kind of acid site, the reduction ability and then influence the catalytic performance of the materials. Mesoporous MnO2 templated from KIT-6 exhibited better catalytic performance than other mesoporous MnO2, indicating that the mesostructure of the materials has significant influence on the NH3-SCR performance. The KIT-6 templated MnO2 could induce 100% NOx conservation ratio range from 75 °C to 275 °C and exhibit good resistance of SO2 and H2O.

In situ DRIFTS study of O3 adsorption on CaO, γ-Al2O3, CuO, α-Fe2O3 and ZnO at room temperature for the catalytic ozonation of cinnamaldehyde

In situ DRIFTS were conducted to identify adsorbed ozone and/or adsorbed oxygen species on CaO, ZnO, γ-Al2O3, CuO and α-Fe2O3 surfaces at room temperature. Samples were characterized by means of TG, XRD, N2 adsorption–desorption, pyridine-IR, nitrobenzene-IR, chloroform-IR, and CO2-TPD. Pyridine-DRIFTS measurements evidence two kinds of acid sites in all the samples. Nitrobenzene, chloroform-DRIFTS, and CO2-TPD reveal that there are large amounts of medium-strength base sites on all the metal oxides, and only CaO, ZnO, and γ-Al2O3 have strong base sites. And the benzaldehyde selectivity was increased in the same order of the alkalinity of the metal oxides. With weaker sites, ozone molecules form coordinative complexes bound via the terminal oxygen atom, observed by vibrational frequencies at 2095–2122 and 1026–1054cm−1. The formation of ozonide O3− at 790cm−1, atomic oxygen at 1317cm−1, and superoxide O2− at 1124cm−1 was detected; these species are believed to be intermediates of O3 decomposition on strong acid/base sites. The adsorption of ozone on metal oxides is a weak adsorption, and other gases, such as CO2, will compete with O3 adsorption. The mechanism of cinnamaldehyde ozonation at room temperature over CaO shows that cinnamaldehyde can not only be oxidized into cinnamic acid, but also be further oxidized into benzaldehyde, benzoic acid, maleic anhydride, and ultimately mineralized to CO2 in the presence of O3.

Shape-selective catalysis by phosphate modified ZSM-5: Generation of new acid sites with pore narrowing

Pore narrowing of ZSM-5 was carried out by phosphate modification technique and shape selectivity was evaluated for alkylation, disproportionation reactions and competitive reaction of ethylbenzene and meta-xylene. Catalysts were characterized by XRD, FTIR, MAS NMR, NH3-TPD, PZC etc. Post synthesis modification of ZSM-5 by phosphate treatment generated new kinds of acid sites with pore narrowing. Even though there was a change in acid site strength, these active sites efficiently catalyzed toluene alkylation with methanol, ethylbenzene alkylation with ethanol and disproportionation of ethylbenzene. These active sites formed monolayer islands over catalyst surface and narrowed the pore opening resulting in high para-selectivity. Decrease in pore volume by phosphate modification has a direct correlation with increase in shape selectivity. Constraint index for modified catalysts obtained from competitive reaction of ethylbenzene and m-xylene was used to determine extent of pore narrowing by phosphate modification.

Effect of rehydration on structure and surface properties of thermally treated palygorskite

Palygorskite has a fibrous like morphology with a distinctive layered appearance. The simplified formula of palygorskite (Mg5Si8O20(OH)2(OH2)4⋅nH2O) indicates that two different types of water are present. The dehydration and rehydration of palygorskite have been studied using thermogravimetry and H2O-temperature programmed desorption. X-ray diffractograms, NH3 adsorption profiles, and NH3 desorption profiles were obtained for thermally treated palygorskite as a function of temperature. The results proved water molecules were mainly derived from Si–OH units. In addition, five kinds of acid sites were found for palygorskite. The number of acid sites of external surfaces was larger than that of the internal surfaces. Bonding on the internal surface acid sites was stronger than the bonding of the external surfaces. Rehydration restored the folded structure of palygorskite when thermal treatment temperature was lower than 300°C.

Preparation of Mesoporous Alumina Particles by Spray Pyrolysis and Application to Double Bond Migration of 2-Butene

The objective of the present study is to investigate the catalytic performance of mesoporous alumina that were prepared via spray pyrolysis for double bond migration from 2-butene to 1-butene. The mesoporous alumina particles were prepared via spray pyrolysis by changing the types of organic surfactants and Al precursors. The texture and acidic properties of mesoporous alumina were analyzed through N2 adsorption, SEM, ammonia-temperature programmed desorption, and FT-IR of adsorbed pyridine. The morphologies and texture properties of the mesoporous alumina were found to have been strongly influenced by the combination of the Al precursor and the structure-directing agents. The mesoporous alumina samples had two kinds of acidic sites: a Lewis acid site and a H-bonded weak acid site. 1-Butene was produced selectively through double bond migration of 2-butene over all of the mesoporous alumina catalysts. The catalyst prepared by using a chloride compound as an aluminium precursor and CTAC as a structure-directing agent showed the highest activity in the double bond migration of 2-butene, which was attributed to its large surface area and an overall high amount of acid sites.

Towards understanding sodium effect on USY zeolite

Samples of a commercial USY zeolite were exchanged with NaCl solution and then thermally treated under dry atmosphere. Resulting samples were evaluated by n-butane cracking, isopropylamine ( i-pam) decomposition, TG–DSC, nitrogen adsorption, and powder XRD. Activation barriers for n-butane reactions were determined as a function of sodium load in the samples. The addition of sodium chloride has a marked negative effect upon material crystallinity, but dealumination rate does not seem to be affected, which suggests framework destruction mediated by sodium does not involve aluminum atoms. As sodium was loaded, samples were less able to retain water because of loss in crystallinity as shown by XRD and volume of micropore measurements. Activity to n-butane cracking decreases monotonically with sodium load, but selectivity to cracking, dehydrogenation, and hydride transfer products remained unaltered; that shows sodium chloride effect is not selective to a particular kind of acid site. The Brønsted acid site (BAS) density measured by i-pam decomposition exhibits a volcano-like profile with sodium load; at low sodium contents there is a gain in i-pam accessibility due to structural collapse but at higher loads BAS disappearance overcomes accessibility and density of acid sites decreases. The average pore size increases with sodium load. After thermal treatment in presence of sodium chloride, gates are opened in zeolite structure, allowing n-butane and isopropylamine to reach formerly inaccessible sites; hydrocarbon conversion can occur in spatially constrained environments where stabilization effects lead to low-energy/low-entropy activated complexes and apparent activation energy shows a minimum with sodium chloride load. Sodium chloride has an effect on Y zeolite even in dry conditions at temperatures similar to those predominant in FCC regenerator.

Preferential oligomerization of isobutene in a mixture of isobutene and 1-butene over sodium-modified 12-tungstosilicic acid supported on silica

Modification of 15 wt% H 4SiW 12O 40/SiO 2 with Na + ions was investigated. Na-modified H 4SiW 12O 40/SiO 2 showed higher selectivity for isobutene oligomerization in the preferential oligomerization of isobutene in an equimolar mixture of isobutene and 1-butene than the unmodified one and the selectivity increased with an increase in the Na + ion content. The highest selectivity (∼97%) was achieved by using Na 3HSiW 12O 40/SiO 2. The activity decreased with an increase in the Na + ion content. Unmodified H 4SiW 12O 40/SiO 2 had two kinds of acid sites with different acid strengths (medium and strong acid sites) on the outermost surface as revealed by temperature-programmed desorption of benzonitrile. However, the strong acid sites were eliminated by modification with alkaline metal cations; for example, Na 3HSiW 12O 40/SiO 2 had a negligible amount of strong acid sites. Thus, we concluded that the absence of strong acid sites on the outermost surface is the reason for the high selectivity. Li 3HSiW 12O 40/SiO 2 and K 3HSiW 12O 40/SiO 2 also exhibited high selectivity for isobutene oligomerization with activities comparable to that of Na 3HSiW 12O 40/SiO 2.

Esterification of Stearic Acid with Triethanolamine over Zirconium Sulfate Supported on SBA‐15 Mesoporous Molecular Sieve

AbstractZirconium sulfate supported on SBA‐15 (pore diameter about 6 nm) mesoporous molecular sieve was synthesized. Various techniques, such as nitrogen adsorption–desorption isotherms, X‐ray diffraction, transmission electron microscopy, and temperature‐programmed desorption, were used to characterize the prepared catalysts. It was shown that the catalyst exhibited straight, parallel, and uniform channels, zirconium sulfate had been finely dispersed on the surface of SBA‐15, and most of the zirconium sulfate species were located inside the pore channels. The catalyst contained two kinds of acid sites with different strengths. Zirconium sulfate supported on SBA‐15 was used to catalyze the esterification of stearic acid with triethanolamine and recycling was used. The composition of esteramine was analyzed by reversed‐phase high‐performance liquid chromatography. The analytic results showed that the catalytic activity and selectivity of zirconium sulfate supported on SBA‐15 in the esterification of stearic acid and triethanolamine was better than that of phosphorous acid, zirconium sulfate supported on MCM‐41, and zirconium sulfate supported on SBA‐15.

1H and 31P solid-state NMR of trimethylphosphine adsorbed on heteropolytungstate supported on silica

Trimethylphosphine (TMP) has been used as an NMR probe in order to determine the acidity of Keggin-type 12-tungstophosphoric heteropolyacid (HPW), pure and supported on silica, dehydrated at 473 K. Adsorption of TMP on pure dehydrated HPW leads to the formation of trimethylphosphonium ions (TMPH +) characteristic of the presence of strong Brönsted acid sites. TMP replaces the water molecules lost by dryness and allows the Keggin secondary structure to recover. Silica interacts with TMP by two kinds of acid sites: with weak acid support sites through the isolated silanol groups and with strong Brönsted acid, which lead to the formation of TMPH +, through the hydrogen-bonded silanol groups. Silica only interacts with HPW through its isolated silanol groups.

Study of the deactivation of Co- and Pt,Co-mordenite during the SCR of NO x with CH 4

Co- and Pt,Co-mordenite catalysts were found to lose about 50% of their initial NO-reduction and CH 4-oxidation activity during a long catalytic NO x -SCR test. Adsorption capacities for N 2, X-ray diffraction and Raman spectroscopic results suggested that the activity loss cannot be explained by either mordenite dealumination or extensive Co 3O 4 formation. Infrared spectra of the adsorbed pyridine showed that fresh catalysts have strong Brönsted and Lewis acidity. The Brönsted acidity was attributed to bridging OH groups of mordenite, formed from H 2O by heterolytic dissociation in the electric field around the Co 2+ ions. During the catalytic reaction, a large fraction of both kinds of acid sites was lost. The drop of acidity and catalytic activity was explained by migration of the Co 2+ ions to lattice positions where the ion is not accessible by pyridine and is coordinated by framework oxygen atoms only. High-pressure DRIFTS spectra of adsorbed N 2 and H 2-TPR results provided additional evidence of the connection between the deactivation and cation migration processes.

Alumina-promoted mesoporous sulfated zirconia: A catalyst for n-butane isomerization

Mesoporous sulfated zirconia (MP-ZrO 2) were synthesized hydrothermally using Zr(O- nPr) 4 as zirconium precursor, ammonium sulfate as sulfur source and CTABr as template. Then the as-synthesized mesoporous materials were directly impregnated with aluminum sulfate to give the acidic Al-promoted mesoporous sulfated zirconia. (AS/MP-ZrO 2). The AS/MP-ZrO 2 catalyst was characterized by nitrogen physisorption (BET) for texture properties, by X-ray diffraction (XRD) for confirming the phase and by TEM for particle sizes. The catalytic conversion of n-butane isomerization was measured in a flow reactor. With the addition of a proper amount of aluminum as a promoter, the catalytic behavior for n-butane isomerization at low temperature in flow system was strongly promoted. The increase of activity was determined primarily by the amount of alumina addition and by the temperature of calcination. The highest catalytic performance is for the catalyst prepared at an optimum 3 mol.% Al loading and calcination at 650 °C. The nature of the acidic sites was determined by X-ray photoelectron spectroscopy (XPS) measurements of N 1 s of the adsorbed pyridine. Three kinds of acid sites were identified on the catalyst: a Lewis site, a weak Brønsted site and a strong Brønsted site. The catalytic activities are correlated with the amount of weak Brønsted acid sites. The remarkable activity and stability of the Al-promoted catalysts are due to a balanced distribution of acid sites strength with an enhanced amount of weak Brønsted acid sites.

Characterization and Catalytic Properties of MAPO-36 and MAPO-5: Effect of Magnesium Content

Samples of MAPO-36 (ATS structure) and MAPO-5 (AFI structure) prepared with different magnesium contents and high crystallinity were characterized by several physical chemical techniques and catalytically evaluated using the isomerization of m-xylene as a reaction model. Chemical analysis of the solids indicated a greater incorporation of Mg into MAPO-5 than into MAPO-36 for the same Mg content in the gel. X-ray photoelectron spectroscopy results indicated that in both MAPOs the concentration of Mg at the surface is larger than that in the bulk in the as-synthesized samples, and that during the calcination process a depletion of the surface Mg has taken place. The calcination process of these samples caused Mg extraction from the framework of both materials. Infrared spectroscopy investigations of adsorbed pyridine showed that the acid strength of MAPO-36 is higher than that of MAPO-5, and that a high fraction of the acid sites disappears on calcining the samples in air. Temperature-programmed desorption of ammonia indicated the presence of two kinds of acid sites, the major part being strong acid sites which increase with Mg content. The catalytic activity of MAPO-36 decreases with the Mg content, which is attributed to a crystal size effect. The activity is much higher than that of MAPO-5 when the concentration of Mg in the crystals is small, but the difference decreases as the solids become richer in Mg.

Superacidity and Catalytic Activity of Sulfated Zirconia

The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm-2, and the adsorption heat of ammonia was ca. 200 kJ mol-1. This corresponds to −19 of the H0 function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm-2), an adsorption heat of ca. 160 kJ mol-1, an H0 of −12, and Bronsted nature. The former was active for the Friedel−Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butan...

Transalkylation of the alkylaromatic hydrocarbons in the presence of ultrastable Y zeolites Transalkylation of toluene with trimethylbenzenes

The catalytic properties of Y zeolites, dealuminated by steam stabilization, for the transalkylation of toluene with trimethylbenzene were investigated in a pulse microreactor. Some factors influencing this reaction were taken into account: the reaction temperature, the degree of dealumination, the nature of acid sites and the deposited coke. The transalkylation reaction proceeds with good yields on USY. At bulk silica to alumina ratio's around 5, a higher activity is observed for the transalkylation of toluene than for the disproportionation of trimethylbenzenes. Both HNaY and USY samples were characterized using FTIR and XPS of chemisorbed pyridine. These techniques allowed to detect the presence of the three kinds of acidic sites: the Brønsted acid sites, the Na + cations and the extra framework AlO + species. The effect of these three sites on catalytic results are discussed as well as the reactivity of alkylaromatic hydrocarbons in the reaction network and the reaction mechanism.

Vanadia Catalysts Supported on Mixed TiO2-Al2O3 Supports: Effect of Composition on the Structure and Acidity

Two series of V2O5/TiO2-Al2O3 catalysts were prepared. In the first, a constant loading of vanadium (7.5 wt% V2O5) was deposited on a series of mixed TiO2-Al2O3 supports of various compositions (0-82 wt% Al2O3), whereas in the second series various amounts of vanadium (4.5-10.5 wt% V2O5) were supported on a TiO2-10 wt% Al2O3 support. X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), and IR spectroscopy were used to study the influence of the composition on the structure of the catalysts and on the oxidation state of supported vanadium species. The acidity of the catalysts was studied using IR of chemisorbed NH3. The results showed the presence of surface V4+ species, in addition to V+5 species, on the V2O5/TiO2-Al2O3 catalysts. The ratio V4+/V5+ was found to depend strongly on both the composition of the mixed support and the loading of vanadium. Deposition of vanadium on these mixed carriers results in an increase of the strength of Lewis-acid sites and of the surface concentration of Brønsted-acid sites, whereas the strength of Brønsted- and the surface concentration of Lewis-acid sites are not significantly affected. For constant vanadium loading, the composition of the mixed support strongly influences the surface concentration of both kinds of acid sites, especially in the range 0-10 wt% Al2O3.

Catalytic properties of [formula omitted] superacids in esterification

SO 4 2− Ti-M-O (M = Al, Fe, Sn) solid superacids were used in the esterification of phthalic anhydride with 2-ethyl hexyl alcohol. The catalytic activity of SO 4 2− TiAlSnO (1) is higher than SO 4 2− TiO 2 and SO 4 2− TiAlO solid superacids. In the presence of (1) DOP is prepared in good yield at 175°C over a period of 6 h. The acid strength, surface area and catalytic activity of SO 4 2− Ti-M-O are obviously affected by the preparation conditions. Temperature-programmed desorption of pyridine adsorbed on the catalyst was used to characterize the acid strength distribution of the catalysts. It was found that there were three kinds of acid sites (weak acid, middle acid and superacid) on SO 4 2− TiAlO , and the concentration of the middle-acid site on the catalysts is directly proportional to its catalytic activity for the esterification. Adding Al into SO 4 2− TiO 2 could increase its surface area greatly.

Cerium impregnated H-mordenite as a catalyst for shape-selective isopropylation of naphthalene. Selective deactivation of acid sites on the external surface

The impregnation of cerium is the effective method for the deactivation of external acid sites of H-mordenite. The selectivity of 2,6-DIPN in the isopropylation of naphthalene was enhanced by the impregnation with such a large amount as 30–50 wt.-% of cerium without significant decrease of catalytic activity. The highest selectivity of 2,6-DIPN was achieved up to 70% over a highly dealuminated H-mordenite, (HM(128); SiO 2/Al 2O 3= 128, with 30 wt.-% of cerium. The enhancement of the selectivity is ascribed to the deactivation of external acid sites judging from the activity of the cracking reaction of 1,3,5-triisopropylbenzene. The effective pore radius was not reduced by the modification. The ceria is highly dispersed only on the external surface of H-mordenite without any formation of new kinds of acid sites. The 129Xe NMR observation suggfests that cerium is not in the pores, but on the external surfaces. The deactivation of the external acid sites is a characteristic property for cerium. Lanthanum and neodymium inhibited catalytic activity of the isopropylation because the pores were narrowed by their impregnation. A possible reason of the deactivation is ascribed to the amphoteric property of ceria.