Effect of vanadium on the behaviour of unsulfated and sulfated Ti-pillared clay catalysts in the SCR of NO by NH3

Vanadium supported on unsulfated and sulfated Ti-pillared clays are developed for the selective catalytic reduction (SCR) of nitrogen oxide (NO) by ammonia. The unsulfated Ti-pillared clays (TiM-PILC) were prepared by hydrolysis of titanium methoxide (Ti(OCH₃)₄) with HCl. The same procedure was employed using H₂SO₄ in order to obtain the sulfated Ti-pillared clays (STiM-PILC). All the pillared materials were characterized before and after vanadium addition by chemical analysis, XRD, N₂-physisorption, TGA, NH₃-TPD, H₂-TPR and UV–vis. It was found that the sulfation or the addition of a low amount of vanadium into the TiM-PILC reduces the surface area but improves the acidity of the material. The addition of vanadium to the STiM-PILC decreases the surface area, the number of acid sites and the stability of the sulfate species but enhances the redox properties of the material. The STiM-PILC and the V-TiM-PILC exhibit higher NO removal activity at high temperature than the TiM-PILC. The presence of both sulfate and a low amount of vanadium (2 wt.%) in the TiM-PILC leads to an excellent active and selective catalyst for SCR-NO with NH₃.     

An improved route for the synthesis of Al<Subscript>13</Subscript>-pillared montmorillonite catalysts

The distribution of Al₁₃ pillars and the process of intercalation in montmorillonite can be enhanced through the application of an ultrasonic treatment. This paper describes the results of ultrasonic treatment in the preparation of Al-pillared montmorillonite with and without prior exchange with Na⁺. The resulting materials have been characterised by X-ray diffraction (XRD), N₂ adsorption/desorption, Scanning Electron Microscopy and Atomic Force Microscopy (AFM). The catalytic activity was tested with the n-heptane hydroconversion test. Optimum results were obtained after ultrasonic treatment up to 20 min without prior Na-exchange before the Al₁₃ intercalation. Longer ultrasonic treatment resulted in partial destruction of the pillared structure. The pore size diameter also increased with increasing ultrasonic treatment up to 20 min with values in the range of 4 nm. This behaviour can be explained by the loss of the typical house of cards structure after prolonged ultrasonic treatment. AFM showed that the pillars in the interlayer of the montmorillonite resulted in a distortion of the tetrahedral sheets of the clay. At atomic scale resolution it was clear that the pillar distribution is not homogenous, confirming earlier results using high resolution TEM. The effects of ultrasonic treatment on the catalytic activity is rather limited, although the pillared clays prepared with short ultrasonic treatments of 5 and 10 min performed slightly better.     

Synthesis and Characterization of Pillared High Layer Charged Synthetic Saponite

Alumina pillared clays were prepared using synthetic saponites with different layer charges, and their cracking activities were characterized by cumene conversion reaction. It was found that the pillared saponite with super high layer charges (2.0) exhibited stable cracking activity in the thermal treatment temperature range from 500 to 700°C. The BET surface areas of the pillared saponites (calcined at 500°C for 2 h) with layer charges of 1.0, 1.5 and 2.0 were 360, 280 and 310 m²/g respectively, according to the N₂ adsorption-desorption determinations at 77 K. A significant layer charge effect was found on the catalytic cracking activity of the pillared clays according to the model reaction, indicating that substitutions in the tetrahedral sheets played an important role in the development of surface Brönsted acidic sites of pillared clay. Because of strong Brönsted sites of pillared saponites, peaks centered at 432°C were found in the spectra of the temperature programmed desorption of ammonia. The structure of pillared saponite with 2.0 layer charges, being the best solid acid catalyst in this study, was also characterized by ²⁷Al MAS NMR. A new peak centered at 1.9 ppm after pillaring was observed, which was ascribed mainly to the octahedral aluminum in the pillar.     

A comparison of the thermal stabilities Ga13, GaAl12 and Al13-pillared clay minerals

A comparison has been made of the thermal stabilities of pillared clays prepared through the intercalation of Ga₁₃, Al₁₃ and GaAl₁₂ polyoxocations by STx-1 montmorillonite. The stabilities, as revealed through differential thermal analyses, powder X-ray diffraction and surface area studies, were found to increase in the order Ga₁₃-PILC < Al₁₃-PILC < GaAl₁₂-PILC. This is in agreement with the order of the stabilities of these polyoxocations in solution, which also correlates with their relative degrees of symmetry, as previously revealed by NMR studies.     

Acid- and base-treated Fe3+-TiO2-pillared clays for selective catalytic reduction of NO by NH3

Fe³⁺-ion-exchanged delaminated pillared clays (PILCs) have been found previously to be more active than the vanadia-based catalysts for selective catalytic reduction (SCR) of NO by NH₃. The effects of acid treatment of the clay (before pillaring) and base treatments of the TiO₂-PILC (before ion exchange) on the activities of the Fe–TiO₂-PILC catalysts were studied. It was found that the acid treatment increased the activity (by 33%), but the base treatments decreased the activity (although they increased the cation exchange capacity of the pillared clay and, hence, the Fe content). The activities of the catalysts were directly related to their surface Brønsted acidities as identified by FT-IR of chemisorbed NH₃.     

Selective catalytic reduction of NO by NH3 on titanium pillared montmorillonite

The acidity of a titanium pillared montmorillonite (Ti-PILC) has been modified by two methods of sulfation. The acidic properties of these pillared clays have been studied by temperature-programmed ammonia desorption and FTIR analysis of pyridine adsorption. The catalytic activity of these titanium pillared clays in the selective catalytic reduction (SCR) of NO by NH₃ with or without SO₂ has been investigated. For the three titanium pillared clays, a high resistance to SO₂ presence has been observed.     

Vermiculites intercalated with Al2O3 pillars and modified with transition metals as catalysts of DeNOx process

Vermiculites intercalated with alumina pillars and modified with transition metals (Cu, Fe) were studied as catalysts of selective reduction of NO with ammonia. Prior to the pillaring process, a raw vermiculite was treated with a solution of nitric acid and then citric or oxalic acid in order to reduce the overall charge of layers. This modification was necessary for a successful pillaring of the clay. Transition metals (Fe, Cu) were deposited on the surface of the modified vermiculites by an ion-exchange method. The obtained samples were characterized with respect to composition (EPMA), structure (XRD), texture (BET) and chemical nature of deposited transition metal species (UV–vis–DRS). The vermiculite based materials have been found to be active and selective catalysts of the DeNOx process. The Cu-containing samples were catalytically active at lower temperatures than the pillared clays modified with iron. A side reaction of ammonia oxidation by oxygen decreased the effectiveness of the DeNOx process in the high temperature range.     

Determination of porosity in pillared clays by N2 adsorption isotherms

N₂ adsorption isotherms of various pillared montmorillonites (PILCs) were analyzed for evaluation of their porosities. The quantitative data of the total and micropore volumes were calculated using the B.J.H. method and the t-plot, respectively. The volume of mesopores is the difference between the total volume and the micropore volume. The linear branch above the P/P ₀ of 0.5 should be used for the calculations of the micropore volume and external surface area.The evaluation of the specific surface area (SA) of the PILCs and influences of the porosity on the calculations were discussed in detail. An upper limit of the monolayer capacity of a porous solid is proposed, based on the adsorption on a nonporous solid. Due to the space restriction in the fine slit-like pores of the PILCs, the specific surface areas calculated with the B.E.T. equation using the adsorption data in a relative pressure region ranging from 0.01 to 0.1 are more accurate.The mean micropore widths of the PILCs, derived from the data of the multi-point B.E.T. SA and micropore volume consist reasonably with the pore widths obtained by XRD diffraction.The step-like curves of the t-plot in low-pressure region and of the logarithmic plot reveal presence of pores of various sizes in the pillared clays. The micropore size distribution can be derived by a new method using the N₂ adsorption data.     

Chlorophyll adsorption by alumina-pillared acid-activated clays

Clays, acid-activated to an optimum level, have been pillared with alumina to give semi-crystalline expanded materials with surface acidities, pore volumes and average pore diameters generally higher than those of the corresponding pillared materials derived from a clay matrix not previously acid-activated. The chlorophyll adsorption capacity of the pillared acid-activated materials is significantly greater than that of pillared, nonactivated clays. The procedures used in the preparation of these pillared acid-activated clays (i.e., temperature of pillaring, method of drying and calcination temperature) have a significant influence on chlorophyll adsorption capacity because they influence both the physical and the chemical properties of the final pillared material. This variation provides a useful means of relating the various properties of the pillared materials to the chlorophyll adsorption capacity. As a result, a correlation has been demonstrated between adsorption capacity and a combination of pore volume and number of strong acid sites (of strength pK_a < −1.5) present in the pillared material. Optimal adsorbents were obtained from freeze-dried samples prepared by exchange at 20°C and calcination in air at 500°C.     

Experimental investigation of sheet flexibility of layered double hydroxides: One-pot morphosynthesis of inorganic intercalates

The sheet flexibility of layered double hydroxides (LDHs) has been investigated experimentally using co-precipitation and urea hydrolysis methods in an aqueous solution of long-chain anion surfactant in this work. Using dodecylsulfate (DS) anion as morphology-controlling agent, layer-bended or contorted Mg/Al-LDH is obtained successfully. The morphology of bent layers is retained during either in situ decomposition of interlayer DS to SO₄²⁻ or ion exchange of interlayer DS by CO₃²⁻. The direct synthesis of the layer-distorted LDHs intercalated with small inorganic anions is quite difficult. It has been achieved using layer-bended LDHs pillared with bulky organic anions as precursors in this paper. The morphosynthesis is expanded to Co/Al and Ni/Al-LDHs, indicative of the general flexibility of this kind of anionic clays.     

Esterification of stearic acid with p-cresol over modified Indian bentonite clay catalysts

The esterification of stearic acid with p-cresol using modified Indian bentonite clay catalysts has been reported. The reaction was studied over exchanged clays, acid activated clays, exchanged acid activated clays, aluminium pillared clay, aluminium pillared acid activated clay, molecular sieve Al-MCM-41, zeolite Hβ, ZrO₂, S-ZrO₂, p-TSA, montmorillonite K10, and montmorillonite KSF in o-xylene for 6 h. The catalysts were characterized by X-ray diffraction and surface area measurements. The acidity was determined by n-butylamine back-titration method and DRIFTS after pyridine adsorption. Acid activated Indian bentonite (AAIB) was found to be a better catalyst compared to other catalysts in the esterification of stearic acid with p-cresol.     

Influence of Synthesis Conditions and Cerium Incorporation on the Properties of Zr-Pillared Clays

Zr-pillared clays were prepared from ZrOCl₂ pillaring solutions by adopting different preparative conditions. Ce³⁺ ions are introduced to Zr-pillared clays by co-intercalation method. The resulting samples were characterized by XRD, TGA, N₂ sorption and UV-VIS-Diffuse reflectance spectroscopy techniques. Basal spacings in the range of 18–21 Å were observed depending upon the preparative condition. TG analysis shows three weight loss regions corresponding to removal of various types of water molecules. All pillared clays show Type-I sorption isotherm typical of microporous materials. Pillaring under refluxing condition is found to have beneficial effect on the surface area and pore volume of the Zr-pillared clay. The chemical environment and location of Ce³⁺ ions is studied by UV-VIS-DRS. The Ce³⁺ ions are found to be present in the micropores of the Zr-pillared clays. However heat treatment at higher temperature may result in peripheral interaction between Ce³⁺ ions and Zr-pillars. Catalytic activity of these pillared clays was evaluated for cyclohexanol dehydration which correlates well with the Brønsted acidity of these materials. The Zr-Pillared clay containing Ce³⁺ ions show good catalytic activity and stability with reaction time which has been ascribed to the stabilazition of the Brønsted acidic centers.     

Cerium Containing Al- and Zr-Pillared Clays: Promoting Effect of Cerium (III) Ions on Structural and Catalytic Properties

Ce³⁺ containing Al- and Zr-pillared clays were prepared by co-hydrolysis and co-intercalation methods, respectively. The materials were characterized by XRD, IR and near IR techniques. The expansion in the clay lattice as a result of pillaring was confirmed from X-ray diffraction study. The basal spacing for the pillared clay materials was observed typically in the range of 18–23 Å. The IR and near IR studies were employed to show the presence of acidic and non-acidic hydroxyl groups in the pillared clays. The IR peak at 3630 cm⁻¹ corresponding to the structural hydroxyl groups was more intense in pillared clays than in the parent clay. This peak is retained even after calcination at 500^∘C. Various overtone and combination bands were observed in the NIR range of 1200–2500 nm for the pillared clay materials. Pillaring with Al- and Zr-inorganic polycations enhanced the catalytic activity of the parent clays for dehydration of cyclohexanol. The presence of Ce³⁺ ions in the pillared clays was found to influence catalytic activity and stability of the Brønsted acidic sites. The stabilizing effect of cerium ion in the pillared clays is attributed to the competitive site occupancy and prevention of octahedral migration of the catalytically active protons.     

Ceria–zirconia-supported rhodium catalyst for NOx reduction from coal combustion flue gases

The catalytic activities of ceria–zirconia mixed oxides Ce_xZr_1−xO₂ (x = 0.17, 0.62 and 0.8) rhodium catalysts were determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNO_x reaction in the presence of the mentioned gas mixture. However, their catalytic activity varies with the content of cerium and goes through a maximum for x = 0.62, leading to 27% NO_x consumption. The effect of rhodium on Ce_0.62Zr_0.38O₂ considerably improves the catalytic activity during the deNO_x process assisted by hydrocarbons. The rhodium addition decreases by about 34 °C the temperature of NO_x consumption, which goes up to 57%. A mechanism of hydrocarbon (HC) assisted reduction of NO is proposed on ceria–zirconia-supported rhodium catalysts. This mechanism is divided in three catalytic cycles involving (i) the oxidation of NO into NO₂, (ii) the reaction of NO₂ and the hydrocarbons leading to RNO_x species and C_xH_yO_z, and finally (iii) the decomposition of NO assisted by these latter C_xH_yO_z species.     

An Investigation of Catalytic Activity for CO Oxidation of CuO/Ce x Zr1–x O2 Catalysts

A series of CuO/Ce _x Zr_1–x O₂ catalyst powders with different Ce/Zr ratio were prepared via an impregnation method and characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), H₂-Temperature-programmed reduction (TPR) and X-ray photoelectron spectra techniques. The catalytic properties of the catalysts were evaluated by means of a microreactor-GC system. XRD results showed that the addition of CuO had no effect on the crystalline lattice of the support. The structures of the Ce _x Zr_1–x O₂ samples were confirmed by XRD analyses and FT-Raman results. The H₂-TPR profiles for these catalysts had three peaks, which could be attributed to the reduction of three kinds of CuO species, i.e., the highly dispersed CuO, the larger CuO species and the bulk CuO. The TPR analyses and catalytic property tests indicated that the Ce/Zr ratio of CuO/Ce _x Zr_1–x O₂ had an effect on the dispersion degree of CuO and the catalytic activity of the catalysts.     

The use of the logarithmic adsorption isotherm for the determination of the micropore-size distribution

The determination of the micropore-size distribution of microporous materials is of major importance in the field of adsorption and catalysis. This new method to derive the micropore-size distribution for Pillared Clays (PILC) is based on the logarithmic adsorption isotherm.When a N₂-adsorption isotherm is replotted on a log P/P ₀ scale, a curve representing the adsorption potential is obtained. Because the adsorption potential is highly dependent on the pore-size, this curve contains all the information on the pore structure. These curves often exhibit steps, large increases in volume adsorbed by small changes in adsorption potential. These are thermodynamically related to the filling of pores of a certain size. The relative pressures where these steps appear are dependent on the pore-size. Taking the derivative, dV/d log (P/P ₀), the location of these steps becomes more clear. These derivative curves give a lot of information on the pore-size distribution. The presence of several maxima and their sharpness informs about the number of different pore-sizes present and the homogeneity. Assuming that N₂-adsorption in the micropores is ruled by the amount of N₂-layers which can fit in, the micropore range can be subdivided in five pore groups adsorbing 1–5 layers. By locating the main maxima in the derivative curves of pillared clays with known but different slit widths, we can correlate a pore-size range to a certain P/P ₀ range where these pores will fill.In this way a micropore-size distribution based on the adsorption potential can be constructed.     

Synthesis of Al-pillared beidellite and its catalytic activity in the hydroconversion ofn-heptane

Aluminum-pillared clays based on synthetic beidellite have been prepared by exchanging hydrothermally synthesized beidellite with partly hydrolyzed aluminum nitrate solution followed by calcination. It is shown that a catalyst containing 50 wt% amorphous silica and 50 wt% pillared beidellite loaded with 0.8 wt% Pt has ann-heptane hydroconversion comparable to a commercially available silica-alumina and does not significantly deactivate with increasing time on stream (within 14 h).     

The Extraction of 137Cs and 89Sr from Waste Simulants Using Pillared Montmorillonite

ABSTRACT

Two samples of a silica-pillared montmorillonite produced using 3-aminopropyltrimethoxy silane and an alumina-pillared montmorillonite were evaluated for the removal of ¹³⁷Cs and ⁸⁹Sr from a simulated nuclear waste solution and a simulated groundwater, and the results were compared to the parent montmorillonite and two zeolites, AW500 (chabazite) and clinoptilolite. The parent and pillared clays were characterized using x-ray powder diffraction and surface area analysis by nitrogen adsorption/desorption studies. The pillared clays exhibited d-spacings of between 173 and 182 A after calcination, and surface areas ranging from 71 to 264 m²g^-1. Both of the silica-pillared clays and the alumina-pillared clay exhibited excellent K_dsfor ¹³⁷Cs from simulated groundwater with values of 23,650, 23,260 and 144,570 mL/g, respectively. These were far better than the K_ds obtained by clinoptilolite and AW500 which had K_ds of only 14,560 and 9650 mL/g, respectively. None of the pillared clays showed a high selectivity for ⁸⁹Sr from groundwater or ^l37Cs from simulated alkaline tank waste. They did, however, show a slight selectivity for ⁸⁹Sr in the simulated Hanford tank wastes, but this is thought to be due to a precipitation mechanism rather than to ion exchange.     

Effect of Ultrasound on the Structural and Textural Properties of Al–Fe Pillared Clays in a Concentrated Medium

A modification of bentonite with the Al–Fe mixed system in a concentrated medium was performed, assisting the intercalation with ultrasound. The solid pillaring agent and clay powder were placed in a dialysis membrane in contact with a minimum amount of water, and ultrasound was used to promote species exchange. Al–Fe pillared clays (2, 5 and 10 mol% Fe) were characterized by XRF, CEC, XRD, H₂-TPR and N₂ adsorption at 77 K, revealing changes in the structural and textural properties of the modified clays in the presence of ultrasound during the intercalation. The catalytic properties of pillared clays were evaluated by using the catalytic wet peroxide oxidation of phenol in a diluted aqueous medium, showing an activity comparable to that of a solid modified through the conventional method.     

Prevention of photocatalytic deterioration of resins using TiO2 pillared fluoromica

The TiO₂ pillared fluoromica powder was kneaded with polylactic acid resin. The composite showed high photocatalytic activity for degradation of acetaldehyde and toluene gas, especially at the range of 1–3 wt.% pillared mica powder, and this photocatalytic activity was higher than that of resins containing even higher amounts of commercial TiO₂ (P-25, Degussa). The composite test pieces of pillared mica showed smaller photocatalytic deterioration than the samples with P-25 powder in out-door weathering tests. Thus, the TiO₂ pillared clay resin composite shows excellent prevention of photocatalytic deterioration and high photocatalytic activity in comparison with P-25.