Surface Acidity Of Alumina Research Articles

Insights into the Effect of a Microwave Field on the Properties of Modified γ-Alumina: A DFT Study

γ-Alumina is often used as a support for hydrodesulfurization catalysts due to its excellent performance. During the catalytic reaction, the strong surface acidity of γ-alumina can induce a strong interaction between the active phase and the support. The reaction activity of the catalyst can be affected by changing the present mode of the active phase on the surface of the support. The (110) crystal plane, acting as the strongest acidity plane of γ-alumina, was selected for modification. The supports modified with boron and phosphorus were successfully constructed, and the acid strengths were quantified by simulating the adsorption of the relevant probe molecules: pyridine in correlation with surface electronic properties via density functional theory. The surface adsorption energy calculation shows that the boron-modified surface is able to moderately reduce the adsorption capacity of alumina, while that of the surface modified by phosphorus is found to be enhanced over the sites of a tetrahedral coordination structure; however, at the other unsaturated Al sites, this is obviously reduced. The results of introducing electric fields imply that applying horizontal electric fields changes the surface acidity of alumina under the premise of a stable structure. With the enhancement of the horizontal electric fields, the adsorption capacity of tetra-coordination sites on the original surface gradually decreases, while those of the others gradually increases. However, for the boron-modified surface, introducing horizontal electric fields can reduce the adsorption capacity of all sites. Hence, microwave-electric-field-assisted modification of B further reduces the surface acidity of alumina, making it beneficial for deep hydrodesulfurization reactions.

Characterization of the NiSO4 site on a NiSO4-ReOx/γ-Al2O3 catalyst for tandem conversion of ethylene to propylene

A series of NiSO4-ReOx/Al2O3 catalysts were synthesized by a co-impregnation method, tested in the conversion of ethylene to propylene at mild conditions, and thoroughly characterized by different spectroscopy techniques. In particular, the attention was focused on the NiSO4 function, which is directly involved in ethylene conversion to 2-butylene, but also drives the catalyst deactivation. Results shows that the sulfate anions increase the surface acidity of alumina, and simultaneously influence the electronic properties of the Ni sites. Indeed, thermal activation of the catalyst promotes the formation of covalent bonds between the sulfate anions and the Ni2+ or Al3+ cations, while keeping constant the + 2 oxidation state of the Ni sites. Studying the initial steps of the ETP reaction reveals the primary acid-catalyzed formation of branched oligomers. Catalyst deactivation could be due to formation and adsorption of long-chain oligomers, or less interaction of sulfate anions with Ni ions.

Influence of calcination parameters on the properties of alumina as a catalyst support

The preparation history of Al2O3 strongly determines its properties for the application as a catalyst support. The interaction of calcination heating rate and temperature and its influence on the structural, textural, and acidic properties of alumina were studied. Commercial-alumina was calcined at 800C, 900C, 1000C, 1050C, 1100C, and 1150C by 2.5C/min, 5C/min, and 10C/min. By increasing temperature up to 1050C, transition aluminas (, , and ) were reported, but the heating rate accelerated phase transformation and changed the amount of each phase in the samples. At 1100C and 1150C, -alumina appeared. Mesopore alumina with di erent surface areas (265-126 m2/g) was synthesized at calcination temperature from 800C to 1050C; when temperature reached 1100C, the surface area decreased drastically and was around 14 m2/g. Interaction of heating rate on the temperature had minimal e ect on the surface area of samples, but considerable e ect on the acidic properties. The intensity of acidic properties was a ected by heating rate with nonlinear pro les. By increasing the heating rate, the ratio of Al cations occupied tetrahedral per octahedral sites and defect of crystal structures increased, so the surface acidity of aluminas was increased.

A comparative study of different fluorine-containing compounds in the preparation of novel alumina binders with rich Brönsted acid sites

Alumina is commonly used as a catalyst binder together with aluminum sol in modern fluid catalytic cracking (FCC) catalysts. The surface acidity properties of alumina strongly affect the catalytic performance of FCC catalysts. Lewis acid sites tend to produce coke because of their dehydrogenation activity, while Brönsted ones produce less coke. Thus, it is beneficial to convert the surface Lewis acid sites into Brönsted type. Fluorine-containing modifiers have been demonstrated to be effective to generate Brönsted acid sites on alumina surface. However, different types of fluorine-containing compounds may have different modification effects. In this work, three fluorine-containing compounds, ammonium fluoroborate (NH4BF4), ammonium fluorosilicate [(NH4)2SiF6], and ammonium fluoride (NH4F), were tested and compared in the modification of alumina surface acidity. Results show that NH4BF4 and (NH4)2SiF6 perform equally well in the generation of Brönsted acid sites, while NH4BF4 is more effective in the reduction of Lewis acid sites. In comparison, NH4F is not so effective in the generation of Brönsted acid sites as the other two compounds.

Control of surface acidity and catalytic activity of γ-Al 2O 3 by adjusting the nanocrystalline contact interface

Densification of 2–2.5 nm nanocrystals assemblies in the three-level hierarchical structure of γ-Al 2O 3 aerogel significantly increases the surface acidity, as determined using indicator titration, NH 3–TPD, and FTIR of adsorbed pyridine. Thermal treatment at 1073 K and insertion of additional alumina inside the aerogel pores eliminated the slit micropores with shrinkage of nanocrystals assemblies (N 2-adsorption, HRTEM, SEM) increasing the contact interface by a factor of 2. It caused a fivefold increase in alumina surface acidity and the strength of Lewis acid sites with no measurable dehydration (weight loss, TGA). This was attributed to the formation of additional low-coordinated aluminum ions with higher charge in the areas with atomic disorder of high-angle grain boundaries, detected by 27Al MAS NMR, HRTEM, XPS, and XRD. Densification of nanocrystalline alumina aerogel yielded a higher catalytic activity in dehydration of isopropanol; it was 10-fold more active compared with commercial γ-alumina on the catalyst weight basis.

Dehydration of methanol to dimethyl ether over nanocrystalline Al2O3 with mixed γ- and χ-crystalline phases

Abstract Dehydration of methanol to dimethyl ether (DME) was investigated over a wide range of nanocrystalline Al 2 O 3 with mixed γ- and χ-crystalline phases. The catalysts were characterized by X-ray diffraction (XRD), N 2 physisorption, ion-exchange titration, and NH 3 -tempearture programmed desorption (NH 3 -TPD). It was found that γ-Al 2 O 3 catalyst containing 20 wt% of χ-phase exhibited the highest yield (86%) with good stability for DME synthesis. The NH 3 -TPD and ion-exchange titration results revealed that the existence of 20 wt% χ-phase in γ-Al 2 O 3 increased significantly both the density and the strength of surface acidity of alumina.

Performance of La-Ba Co-modified Alumina Prepared by Peptizing Method

La-Ba co-modified alumina was prepared from two kinds of pseudo boehmite and HNO_3 as peptizer by peptizing method.The structure and surface property of the alumina were characterized by XRD,BET surface area analyzer,NH_3-temperature programmed desorpfion and NO_2-temperature programmed desorption technology.XRD results showed thatγ-Al_2O_3 phase existed in all the modified alumina samples after calcination at 1273 K.A little BaCO_3 was generated when content of BaO was increased to 14%(w).BET results showed that both of 5%(w) La_2O_3- modified and 5% La_2O_3-8%BaO co-modified alumina (Ba-8) samples exhibited larger specific surface area and pore volume after calcination at 1273 K.N_2 adsorption and desorption isotherms of all alumina exhibited that their surface pore structure was slit-shape and ink-shape pores,pore diameter distribution of Ba-8 was broader and centered between 5 and 10 nm,and other samples centered at 10 nm.NH_3-TPD results indicated that the larger the BaO doping concentration,the less the acidic amount was.At the same time,surface acidity of alumina apparently became weak. NO_2-TPD results showed that BaO doped alumina adsorbed more NO_2 than La_2O_3 doped,sample Ba-8 adsorbed the most NO_2.As sample Ba-8 simultaneously exhibited good textural properties,proper surface acidic amount,acidity and stronger NO_2 adsorption capacity,it had the best catalytic performance when it was used as catalyst carder.The temperatures of light-off and complete conversion of C_3H_3 were 526 K and 593 K,respectively.

Effect of temperature on the acid–base properties of the alumina surface: Microcalorimetry and acid–base titration experiments

Sorption reactions on natural or synthetic materials that can attenuate the migration of pollutants in the geosphere could be affected by temperature variations. Nevertheless, most of the theoretical models describing sorption reactions are at 25 °C. To check these models at different temperatures, experimental data such as the enthalpies of sorption are thus required. Highly sensitive microcalorimeters can now be used to determine the heat effects accompanying the sorption of radionuclides on oxide–water interfaces, but enthalpies of sorption cannot be extracted from microcalorimetric data without a clear knowledge of the thermodynamics of protonation and deprotonation of the oxide surface. However, the values reported in the literature show large discrepancies and one must conclude that, amazingly, this fundamental problem of proton binding is not yet resolved. We have thus undertaken to measure by titration microcalorimetry the heat effects accompanying proton exchange at the alumina–water interface at 25 °C. Based on (i) the surface sites speciation provided by a surface complexation model (built from acid–base titrations at 25 °C) and (ii) results of the microcalorimetric experiments, calculations have been made to extract the enthalpic variations associated respectively to first and second deprotonation of the alumina surface. Values obtained are Δ H 1 = 80 ± 10 kJ mol −1 and Δ H 2 = 5 ± 3 kJ mol −1 . In a second step, these enthalpy values were used to calculate the alumina surface acidity constants at 50 °C via the van't Hoff equation. Then a theoretical titration curve at 50 °C was calculated and compared to the experimental alumina surface titration curve. Good agreement between the predicted acid–base titration curve and the experimental one was observed.

Effect of Aluminum Hydroxide Precipitation Conditions on the Alumina Surface Acidity

The effect of specific ions present during the precipitation of alumina precursor (aluminum hydroxide) on resulting alumina surface acidity has been investigated. It was found that both the method of pH control and the nature of ions introduced to the slurry apparently affected the phase composition of aluminum hydroxide as well as the strength and distribution of alumina active sites measured by means of the temperature programmed desorption of ammonia (NH{sub 3} TPD). The results were interpreted in terms of processes taking place during both polycondensation of aquo ions and aging of amorphous hydrogel of aluminum hydroxide. The ions present in the medium of precipitation (particular at the beginning of aluminum hydroxide precipitation) apparently influenced the course of the reaction.

Surface Characterization of Modified Aluminas: III. Surface-Features of PO 4-Doped Al 2O 3

Pure and PO 4-doped aluminas (≍ 3% P 2O 5), pretreated at three different temperatures (773, 1273, and 1473 K), have been compared by XRD, TEM, and FTIR spectroscopy. The addition of phosphates does not modify the phase transition of low-temperature spinel aluminas (γ-Al 2O 3) to high-temperature spinel aluminas (δ, θ-Al 2O 3), and delays somewhat the phase transition from spinel alumina to the corundum phase (α-Al 2O 3). Phosphates have a positive effect on surface area and porosity only for the corundum phase obtained at T ≥ 1450 K, in which the particles morphology is modified with respect to pure (α-Al 2O 3). The effect of PO 4-doping is appreciable on surface basicity and acidity. The weak basicity of alumina is gradually eliminated, with increasing firing temperature. The presence of phosphates increases the strong surface acidity of aluminas: phosphates tend to collect preferably on the flat patches of regular crystal planes, and so doing decrease the amount and increase the strength of the Lewis acid sites (coordinatively unsaturated (cus) Al IV ions) present on the regular crystal planes. Meanwhile, the presence of phosphates produces an appreciable increase of the number of strong cus Al IV Lewis acid sites present in crystallographically and/or coordinatively defective configurations. When the bulk transition to α-Al 2O 3 has occurred in systems treated at T ≥ 1400 K, the samples retain surface properties reminiscent of those of the transition alumina phases. The diverse opinion of investigators that phosphates act toward alumina as phase stabilizing needs some corrections and additions: the positive role of phosphates on the alumina support implies the stabilization of higher amounts of the strong Lewis acid sites that are most likely to be important in catalytic applications.

Effect of surface acidity of alumina on the direction of conversions of 1-propenylnaphthalene

Effect of surface acidity of alumina on the direction of conversions of 1-propenylnaphthalene