Biodiesel fuel (fatty acid esters) has become more and more attractive due to its environmental benefits. Transesterification is the most common and important method for making biodiesel from vegetable oils or animal fats. Several studies have focused on the development and improvement of analytical methods for monitoring biodiesel production and determining the fuel quality. Analytical procedures reported in the literature include chromatographic methods (e.g., gas chromatography, high-performance liquid chromatography, gel permeation chromatography, etc.) and spectroscopic methods [e.g., 1H and 13C NMR, near infrared, Fourier transform infrared spectroscopy, and recently, Fourier transform (FT)-Raman]. The study presented in this paper expands our previous research, in which FT-Raman spectroscopy combined with partial least squares (PLS) multivariate analysis was successfully applied to the quantification of soybean oil/ethyl ester mixtures. The FT-Raman/PLS methods developed by our group were used to monitor and quantify a transesterification reaction process involving soybean oil and ethanol to produce fatty acid ethyl esters (biodiesel) over 22 h catalyzed by a heterogeneous Lewis acid catalyst. The results were successfully correlated with two 1H NMR spectroscopic methods reported in the literature and a new 1H NMR method proposed in this work that can be easily extended to other vegetable oils. The correlation coefficients (R 2) obtained from the linear fit between FT-Raman measurements and the above 1H NMR methods were 0.9974, 0.9847, and 0.9972, respectively.

The cal-ad analysis of HZSM-5 provides a novel characterization of the donor-acceptor properties of this solid acid showing 0.0415 mmol per g of a strong Brönsted site and 0.53 mmol per g of a weaker site. Of all studies to date, only cal-ad provides a measure of the extent, Ki ; strength, −ΔHi ; and quantity of sites. The relative K values show why gas phase calorimetry, TPD, TGA, DSC, and isopropylamine decomposition do not resolve the two sites found by cal-ad and incorrectly suggest 0.6 mmol g-1 of strong acid sites. Reaction with pyridine gives an enthalpy of 42.1 and 8.6 kcal mol-1, respectively, for the two sites suggesting the latter is a hydrogen bonding site. Other acidity measures average these sites, and their enthalpies contain dispersion as well as donor-acceptor components. Comparison of these results with other cal-ad results suggests HZSM-5 is a strong solid acid but not a super acid. Titrations with 2,6-lutidine and 2,6-di-tert-butylpyridine indicate that the strong Brönsted sites are located in the 5.5 Å channels with no detectable amounts on the exterior surface.

No abstract

No abstract

This report investigates the causes of the unique properties and reactivity of the titanium silicalite TS-1, a commercial zeolite containing 1% Ti by weight. The combined analysis of calorimetric and adsorption data, cal−ad, for the reaction of TS-1 with pyridine shows the absence of the strong acid site found in HZSM-5 and the presence of only 0.07 mmol g-1 of a −15.1 kcal mol-1 hydrogen-bonding site. Titrations with 2,6-lutidine indicate the same number of hydrogen-bonding sites with a lower enthalpy (−9.8 kcal mol-1). The magnitude of these enthalpies indicate that the acceptor sites are hydrogen-bonding sites that are comparable in acidity to the strongest sites on silica gel. The small amounts of these sites, 0.07 mmol g-1 compared to 0.8 mmol g-1 in silica gel and 0.5 mmol g-1 for the hydrogen-bonding sites of HZSM-5, account for the lower affinity of TS-1 for water. The absence of strong acidity prevents epoxide ring opening and explains the utility of TS-1 in epoxidation. Comparison of the results from the multiple equilibrium analysis of gas-phase adsorption isotherms for CO and CH4 by TS-1, HZSM-5, and silica gel indicates that the surface of TS-1 is not more polarizable than the other solids. Thus, the porosity and small number of hydrogen-bonding-sites enables TS-1 to adsorb hydrocarbons in the presence of water.

No abstract

No abstract

Supported niobium pentoxide materials have been effective catalysts for a variety of acid and redox reactions (e.g., dehydration of alcohols, esterifications, etc.). To the best of our knowledge, there are no reports about Nb 2 O 5 supported on silica-alumina. Catalysts of Nb 2 O 5 /SiO 2 -Al 2 O 3 were prepared with 2, 5, 10, 15, 20, and 25 wt % of Nb 2 O 5 by aqueous solution impregnation using ammonium niobium oxalate on silica-alumina. The materials, after being dried at 100 °C, were calcined at 800 °C and characterized by several methods. Investigation through X-ray diffraction showed the typical patterns of crystalline Nb 2 O 5 , which were composed of mixtures of orthorhombic and monoclinic phases (T, M, and H, respectively) present in the materials with content higher than 10 wt %. DTA curves displayed an exothermic peak at 1356 °C (average) without mass loss (confirmed by TG), which may be ascribed to a phase transition (H phase formation) of Nb 2 O 5 supported on silica-alumina. Pure Nb 2 O 5 ‚nH 2 O showed a transition from amorphous to hexagonal or orthorhombic phase (TT or T, respectively) at 567 °C. FTIR and DRIFTS results confirmed the reaction of the niobium oxide with the hydroxyl functionality of silica-alumina. The formation of surface niobium pentoxide species over the support through selective and progressive consumption of hydroxyl groups from the support and the appearance of characteristic niobium hydroxylated species on the surface were demonstrated. Raman spectra attested a two-dimensional overlayer of niobium pentoxide on silica-alumina at contents below 10 wt %. At higher concentrations, the absorptions characterize the formation of phases T and H of bulk Nb 2 O 5 .