Biodiesel production from sewage sludge using supported heteropolyacid as heterogeneous acid catalyst.

Phosphotungstic acid (HPW) and silicotungstic acid (HSiW) were tested as homogeneous and as heterogeneous catalysts (after immobilized on different supports as high surface area graphite -HSAG500-, montmorillonite -MMT- and alumina -Al2O3-) for the in situ transesterification of sewage sludge lipids. Both catalysts exhibited similar performance in homogeneous phase, with slightly higher biodiesel yield for HPW. When the different supports were tested with HPW, the maximum yield obtained follow the trend: MMT > HSAG500 > Al2O3, but a greater leaching of the heteropolyacid (HPA) was observed with MMT. Therefore, HSAG500 showed the best results with a good FAMEs profile. The percentage of active phase was optimized from 1 to 40%, reaching the optimum at 10%. A more heterogeneous surface is obtained with larger quantities, also favouring the HPA leaching. The reaction temperature and the use of sonication as pre-treatment were also optimized. The best results were obtained after sonication with HPW-HSAG500 (10%) as catalyst, catalyst/sludge ratio 1:2, MeOH/sludge ratio 33:1, 120 °C and 21 h of reaction time with a maximum biodiesel yield of 31.1 % (FAMEs/lipids). In view of the results obtained HPW supports on HSAG500 offers a novel alternative as heterogeneous acid catalyst for in situ transesterification using sewage sludge as raw material.

Dolomite industrial by-products as active material for CO2 adsorption and catalyst for the acetone condensation.

The feasibility of using dolomite powders, by-product from the refractory industry, as a CO2 adsorbent and as a catalyst for the acetone liquid-phase self-condensation is demonstrated in this article. The performance of this material can be largely improved by combining physical pretreatments (hydrothermal ageing, sonication) and thermal activation at different temperatures (500-800 °C). The highest CO2 adsorption capacity was observed for the sample after sonication and activated at 500 °C (46 mg·g-1). As to the acetone condensation, the best results were obtained also with the sonicated dolomites, mainly after activation at 800 °C (17.4% of conversion after 5 h at 120 °C). The kinetic model reveals that this material optimizes the equilibrium between catalytic activity (proportional to the total basicity) and deactivation by water (specific adsorption process). These results demonstrate that the valorisation of dolomite fines is feasible, proposing attractive pretreatments for obtaining activated materials with promising results as adsorbents and basic catalysts.

Influence of delignification and reaction conditions in the aqueous phase transformation of lignocellulosic biomass to platform molecules.

The effect of oxidative and reductive delignification processes on the hydrolysis of pine sawdust at mild conditions (200-1000 ppm of HCl and 140-220 °C) is studied in this work. Dimers and reduced sugars are the main products obtained with the fresh sawdust (>82%), reaching a maximum liquid phase yield of 17% after 8 h, at the strongest conditions. This conversion increases up to almost 40% with the pretreated sawdust, obtaining selectivities higher than 87% of levulinic acid and a well-defined distribution of the relevant platform molecules (sugars, HMF, furfural, levulinic acid) as function of the severity of the reaction, decreasing the humins formation and being possible to define different conditions to maximize each yield. These conclusions were corroborated by the kinetic analysis, obtaining a clear decrease in the energy activation for all the individual steps involved in this process.

Carbon Materials as Phase-Transfer Promoters for Obtaining 5-Hydroxymethylfurfural from Cellulose in a Biphasic System.

Different carbonaceous materials were tested as mass-transfer promoters for increasing the yield of 5-hydroxymethylfurfural (5-HMF) in biphasic cellulose hydrolysis. The benefits of working with a biphasic system (water/methyl isobutyl ketone) under soft acid conditions were taken as starting point (no humins or levulinic acid production), with slow extraction kinetics as the weakest point of this approach. Carbon nanotubes (CNTs) and activated carbon (AC) were proposed to improve 5-HMF liquid-liquid mass transfer. A kinetic analysis of the extraction process indicated the competition between 5-HMF and glucose adsorption as the main cause of the poor results obtained with AC. In contrast, very promising results were obtained with CNTs, mainly at 1.5 wt % loading, with complete transfer of HMF and a high global mass-transfer coefficient. The use of CNTs improved the amount of 5-HMF in the organic phase by more than 270 %.

Effect of sewage sludge composition on the susceptibility to spontaneous combustion.

The different technologies applied to the sewage sludge management have in common a first step devoted to the storage. In the case of dried sludges, this storage leads to important safety concerns because of the explosive character of the resulting dusts. In order to ensure safety in the storage step, it is necessary to evaluate the spontaneous combustion trends on terms of measurable chemical and physical properties of the dried sludges. In order to accomplish this scope, twelve samples from different wastewater treatment plants were characterized, correlating the susceptibility to spontaneous combustion with both the sludge composition and the heating value. Equations traditionally used for coals were used to determine the higher heating value from the chemical composition, finding as main source of error the high oxygen content of the sludge samples. Concerning the thermal susceptibility, different parameters were obtained (Maciejasz Index, induction temperature, maximum weight loss temperature, characteristic temperature and activation energy), being in all cases the spontaneous combustions favored by high H/C and low O/C ratios. Likewise, the presence of sulphur in the dried sludge was found to increase the thermal susceptibility of the material. This effect is tentatively explained with the formation of pyrophoric iron sulfides.

Aqueous-Phase Transformation of Glucose into Hydroxymethylfurfural and Levulinic Acid by Combining Homogeneous and Heterogeneous Catalysis.

Homogeneous (HCl) and heterogeneous catalysts (various zeolites) were combined to enhance the upgrading of glucose into two different platform molecules: 5-hydroxymethylfurfural (HMF) and levulinic acid (LA). ß-Zeolite was the most active material for the glucose isomerization to fructose, improving also the activity of HCl for the subsequent steps. Reaction time, temperature, and pH were then modified, identifying 140 °C, 200 ppm of HCl, and 5 h as the optimum conditions for HMF formation (41 % selectivity; 0.06 mol L-1 ) and 140 °C, 400 ppm of HCl, and 24 h for LA formation (34 % selectivity; 0.12 mol L-1 ). This new approach is also relevant because of significant changes to the mechanism for humins formation. All results are successfully fitted to a kinetic model simultaneously considering both catalytic mechanisms.

Effect of sludge features and extraction-esterification technology on the synthesis of biodiesel from secondary wastewater treatment sludges.

Secondary sludge from municipal wastewater treatment plant is proposed as a promising alternative lipid feedstock for biodiesel production. A deep study combining different type of raw materials (sludge coming from the oxic, anoxic and anaerobic steps of the biological treatment) with different technologies (liquid-liquid and solid-liquid extractions followed by acid catalysed transesterification and in situ extraction-transesterification procedure) allows a complete comparison of available technologies. Different parameters - contact time, catalyst concentration, pretreatments - were considered, obtaining more than 17% FAMEs yield after 50min of sonication with the in situ procedure and 5% of H2SO4. This result corresponds to an increment of more than 65% respect to the best results reported at typical conditions. Experimental data were used to propose a mathematical model for this process, demonstrating that the mass transfer of lipids from the sludge to the liquid is the limiting step.

A hydrothermal peroxo method for preparation of highly crystalline silica-titania photocatalysts.

A new completely inorganic method of preparation of silica-titania photocatalyst has been described. It has been established that the addition of silica promotes crystallinity of TiO2 anatase phase. Relative crystallinity and TiO2 crystal size in the silica-titania particles increase with the silica content until SiO2/TiO2 molar ratio of 0.9, but at higher molar ratios they start to decrease. The single-source precursor containing peroxo titanic (PTA) and silicic acids has been proved to be responsible for high crystallinity of TiO2 encapsulated into amorphous silica. It has been proposed that peroxo groups enhance rapid formation of crystalline titania seeds, while silica controls their growth. It has been concluded from the TEM that the most morphologically uniform anatase crystallites covered with SiO2 particles are prepared at SiO2/TiO2 molar ratio of 0.4. This sample, according to (29)Si NMR, also shows the high content of hydroxylated silica Q(3) and Q(2) groups, and it is the most photocatalytically active in UV-assisted decomposition of methylene blue among the tested materials. It has been determined that the increase in the amount of the condensed Q(4) silica in the mixed oxides leads to the decrease in photocatalytic performance of the material, despite its better crystallinity. High crystallinity, low degree of incorporation of Ti atoms in SiO2 in the mixed oxide and adsorption of methylene blue in the vicinity of photoactive sites on the hydroxylated silica have been considered as the main factors determining the high degradation degree of methylene blue in the presence of silica-titania.

One-pot aldol condensation and hydrodeoxygenation of biomass-derived carbonyl compounds for biodiesel synthesis.

Integrating reaction steps is of key interest in the development of processes for transforming lignocellulosic materials into drop-in fuels. We propose a procedure for performing the aldol condensation (reaction between furfural and acetone is taken as model reaction) and the total hydrodeoxygenation of the resulting condensation adducts in one step, yielding n-alkanes. Different combinations of catalysts (bifunctional catalysts or mechanical mixtures), reaction conditions, and solvents (aqueous and organic) have been tested for performing these reactions in an isothermal batch reactor. The results suggest that the use of bifunctional catalysts and aqueous phase lead to an effective integration of both reactions. Therefore, selectivities to n-alkanes higher than 50% were obtained using this catalyst at typical hydrogenation conditions (T=493 K, P=4.5 MPa, 24 h reaction time). The use of organic solvent, carbonaceous supports, or mechanical mixtures of monofunctional catalysts leads to poorer results owing to side effects; mainly, hydrogenation of reactants and adsorption processes.

Improvement on the catalytic performance of Mg-Zr mixed oxides for furfural-acetone aldol condensation by supporting on mesoporous carbons.

A new procedure for improving the performance of the most common catalysts used in aqueous-phase aldol condensation (Mg-Zr mixed oxides) reactions is presented. This reaction is of interest for upgrading carbohydrate feedstocks. The procedure involves supporting Mg-Zr oxides on non-microporous carbonaceous materials, such as carbon nanofibers (CNFs) or high-surface-area graphites (HSAGs), using either incipient wetness or coprecipitation procedures. The use of HSAGs together with the coprecipitation method provides the best performance. Results obtained for the cross-condensation of acetone and furfural at 323 K reveal that the catalyst performance is greatly improved compared to the bulk oxides (96.5 % conversion vs. 81.4 % with the bulk oxide; 87.8 % selectivity for C13 and C8 adducts vs. 76.2 % with the bulk oxide). This difference is even more prominent in terms of rates per catalytically active basic site (four and seven times greater for C8 and C13 adducts, respectively). The improved performance is explained in terms of a more appropriate basic site distribution and by greater interaction of the reactants with the carbon surface. In addition, deactivation behavior of the catalyst is improved by tuning the morphology of the carbonaceous support. An important enhancement of the catalytic stability can be obtained selecting a HSAG with an appropriate pore diameter. With HSAG100 the activity decreased by less than 20 % between successive reaction cycles and the selectivity for the condensation products remained almost unaltered. The decrease is greater than 80 % for the bulk oxides tested at these conditions, with important increases in the selectivity for by-product formation.