Total Surface Acidity Research Articles

Decomposition Reactions of Methanol and Ethanol Catalyzed by Tungsten Oxide Supported on Activated Carbon

Activity of the catalysts to obtain the dehydration products linearly increases with their total surface acidity. Results obtained seem to point out that the surface basicity of the support influences the acid strength of the tungsten oxide particles, especially at low tungsten loading.

Changes in surface chemistry of activated carbons by wet oxidation

A series of activated carbons with different degrees of activation were oxidized with H 2O 2, (NH 4) 2S 2O 8 and HNO 3 in order to introduce different oxygen surface complexes. Changes in the surface chemistry of the activated carbons after their oxidizing treatments were studied by different techniques including temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR), titrations with HCl and NaOH, measurements of the pH of the point of zero charge and catalytic dehydration of methanol. Results showed that treatment with (NH 4) 2S 2O 8 fixed the lowest amount of both total oxygen and surface acid groups. However, this treatment yielded the acid groups with the highest acid strength. This could be because it favors fixation of carboxyl groups close to other groups, such as carbonyl and hydroxyl, which enhances their acidity.

98/00247 Waste plastics liquefaction and coprocessing with coal

Hydrotreating of heavy oil fractions is usually carried out in fixed bed reactors using alumina-supported Ni-Mo and Co-Mo catalysts. The catalytic properties of hybrid Al- and Ti-MCM-41 materials were compared for HDS of thiophene and heavy vacuum gas oil (HVGO) derived from Maya crude. Upon addition of 20 wt.% TiO2, the hexagonal symmetry of the pore arrays was unchanged, but the surface area increased from about 1040 up to 1138 m2/g. In parallel, NH3-thermal programmed desorption (TPD) studies showed an increase of surface acidity with TiO2 content, i.e., from 38 μmol NH3/g for initial MCM-41 to about 62 μmol NH3/g for 80% Ti-substituted Si-MCM-41. Also, Ni-Mo and Ni-W metal loadings increased the total surface acidity, from about 44 μmol NH3/g for NiMo/MCM-41 to about 148 μmol NH3/g for NiMo/MCM-41. Also, the hydrothermal stability of the Ti-MCM-41 materials increased from about 500 °C up to 600 °C under 100% water vapor. Finally, sulfided forms of Ti-MCM-41-supported Ni-Mo and Ni-W catalysts were tested for HDS of heavy vacuum gas oil and thiophene at 350 °C, the NiMo/Ti-MCM-41 and NiW/Ti-MCM-41 catalysts with 50% Ti showing the highest activity, which was comparable to reference catalysts.

Influence of the structure of alumina–aluminium phosphate (AAP) catalysts containing Fe3+and Cr3+on their catalytic activity for isopropyl alcohol decomposition

Phosphate catalysts of the general formula Al100PxM20, where M = Al, Cr, Fe and x= 0, 4.5, 9, 18, 36, 72 and 144, have been prepared by coprecipitation of the corresponding nitrate salts and phosphoric acid with ammonia at pH 9.5. The resulting solids, after characterization by surface area and XRD measurements, were checked for their total surface acidity (Bronsted and Lewis) by pyridine titration and also tested for their catalytic activity towards the isopropanol dehydration. It was found that for the Al100PxAl20 solids the surface density of the acid sites increases on addition of phosphorus in a fairly regular manner. Substitution of some Al by Cr or Fe blurrs the picture of increasing acidity described above. The catalytic activity for isopropyl alcohol dehydration, calculated as the number of molecules decomposed per acid site per second, shows a minimum at x= 18–36 for Al100PxAl20 and Al100PxFe20, while for Al100PxCr20 a continuous increment with x is observed. The activity minima of the first two sets of solids coincide with the total absence of crystallinity as detected by XRD measurements. On the contrary, the continuous increment of activity for the Cr-containing catalysts is related to the presence of crystalline α-Cr2O3. This behaviour is attributed to competition between the diffusional and chemical steps controlling the reaction's process. On the amorphous solids, where high values of activation energy and decreased activity are observed, the chemical reaction seems to be the slow and rate-determining step, while diffusion is fast because of the large porosity of the solids. The other samples show activation energies that are half those of the amorphous solids and larger activities. The controlling step in these cases seems to be diffusion, a fact related either to smaller pores or larger rate constants.

Bleaching with alternative layered minerals: A comparison with acid‐activated montmorillonite for bleaching soybean oil

AbstractAn extensive series of layered minerals including montmorillonite was studied to determine if the fundamental physicochemical properties responsible for pigment adsorption could be identified. Samples were subjected to a uniform preparation regimen to eliminate such secondary effects as particle size, moisture content, level of activation and degree of washing. By doing so, it has been possible to show that both carotene and chlorophyll adsorptions can be described by a rather simple model that employs both surface acidity and pore volume as key variables. Further, it was found that this model provides an adequate fit to the actual data only when specific regions for these variables are considered (i.e., concentration of sites of strong surface acidity, not just total surface acidity; pore volume in region 50\s‐200 Å, not just total pore volume).

Use of Lithium Aluminum Hydride in the Study of Surface Chemistry of Carbon Black

Abstract Considerable interest has been focused in recent years on the effects of carbon black surface chemistry on its properties in rubber and other polymer systems. Methods have been developed in our laboratory for the quantitative characterization and description of the surface chemistry of carbon blacks. The total surface acidity as given by the active (acidic) hydrogen is determined by reaction with strongly basic lithium aluminum hydride in diethyl carbitol, or by equilibration with aqueous alkali under nitrogen. Exclusion of oxygen is essential since carbon blacks readily undergo base catalyzed oxidation. Selective neutralization techniques are employed to further differentiate between strong and weak surface acids. For example, using aqueous sodium bicarbonate solution, only those acids stronger than carbonic acid will be neutralized. Strong acids, as determined by bicarbonate neutralization, are shown to be aromatic carboxyl groups, while weak acids determined by difference between total acidity and strong acid content are phenols and hydroquinone derivatives. Neutral quinone and lactone surface groups are quantitatively determined by chemical reduction with lithium aluminum hydride, or alternatively by analysis of their thermal decomposition products (carbon monoxide and dioxide, hydrogen, and water) obtained on vacuum pyrolysis of carbon black. The distribution of neutral and acidic surface groups on representative channel, furnace, and thermal black is discussed in terms of the state of oxidation of the individual carbons.