Juan A. Melero

Heterogeneous acid catalysts for biodiesel production: current status and future challenges

The reduction of oil resources and the consequent increasing price of oil distillates as well as the environmental concerns of conventional fuels has renewed and increased interest on the preparation of biofuels from renewable resources. One of those interests is nowadays focused on biodiesel, which is usually prepared from crude and refined triglyceride containing raw materials, such as vegetable oils, animal fats and wastes—for instance waste cooking oil and yellow and brown grease. Since several commercial interests converge on this kind of feedstock, one of the priorities being crops for human food supply, the research efforts on biodiesel production are diverting towards the use of low quality triglyceride-containing raw materials. Nevertheless, all of these feedstocks feature high water and free fatty acids (FFAs) content, which strongly affects the behaviour of conventional homogeneous base catalysts. These catalysts are primarily NaOH and KOH, but also NaOCH3 and KOCH3 are employed—as solutions in methanol—mainly in large-scale production plants. In this context, an appropriate solid acid catalyst which could simultaneously carry out esterification of FFAs and transesterification of triglycerides would be of great interest for biodiesel production. Moreover, a heterogeneous acid catalyst could be easily incorporated into a packed bed continuous flow reactor, simplifying product separation and purification and reducing waste generation. The present review attempts to provide a wide overview on the possibility of heterogeneous acid catalysts for biodiesel production replacing the homogeneous conventional process. In this way, three aspects of solid acid catalysis for biodiesel production will be reviewed. The first section deals with the solid acid-catalyzed esterification of FFAs, the second topic relates to the transesterification of triglycerides, while the third deals with solid acid-catalyzed transformation of bioglycerol into oxygenated compounds for biodiesel formulation.

Direct syntheses of ordered SBA-15 mesoporous materials containing arenesulfonic acid groups

SBA-15 mesoporous silica has been functionalized with arenesulfonic acid groups by means of a one-step simple synthesis approach involving co-condensation of tetraethoxysilane (TEOS) and 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) in the presence of a poly(alkylene oxide) block copolymer (Pluronic 123) under acid silica-based catalysis. The resultant materials show hexagonal mesoscopic order and pores sizes up to 60 A, with acid exchange capacities of ca. 1.3 mequiv. H+ per g SiO2 and surface areas up to 600 m2 g−1. The sulfonic groups anchored to the silica surface of the pore walls are thermally stable to temperatures up to 380 °C and resistant to leaching in organic and aqueous solutions under mild conditions. 31P MAS NMR measurements of chemically adsorbed triethylphosphine oxide and the catalytic properties confirm the presence of Bronsted acid centers in these mesoporous materials containing arenesulfonic acid groups that are stronger than those found in propanesulfonic-modified SBA-15 and Al-MCM-41.

Biomass as renewable feedstock in standard refinery units. Feasibility, opportunities and challenges

Within the present contribution we highlight the feasibility of standard refinery units for the production of biofuels from different biomass-derived feedstock. The energy densification of biomass, as well as it’s logistics and incorporation within the refinery supply chain is thoroughly discussed. Likewise, special attention is focused on the catalytic cracking and hydrotreating of triglyceride-rich biomass feedstock, which is probably the most suitable one for co-processing in conventional refinery conversion units. However, the opportunities of other highly oxygenated feedstocks such as pyrolysis oils and sugars are also discussed. Conversion of different feedstocks into conventional liquid fuels by coupling of aqueous phase reforming (APR) with catalytic systems typical of standard petroleum refineries is also evaluated. Thus, here we review the chemistry, catalysis and challenges involved in the production of biofuels from biomass in conventional refineries.

Efficient Isomerization of Glucose to Fructose over Zeolites in Consecutive Reactions in Alcohol and Aqueous Media

Isomerization reactions of glucose were catalyzed by different types of commercial zeolites in methanol and water in two reaction steps. The most active catalyst was zeolite Y, which was found to be more active than the zeolites beta, ZSM-5, and mordenite. The novel reaction pathway involves glucose isomerization to fructose and subsequent reaction with methanol to form methyl fructoside (step 1), followed by hydrolysis to re-form fructose after water addition (step 2). NMR analysis with (13)C-labeled sugars confirmed this reaction pathway. Conversion of glucose for 1 h at 120 °C with H-USY (Si/Al = 6) gave a remarkable 55% yield of fructose after the second reaction step. A main advantage of applying alcohol media and a catalyst that combines Brønsted and Lewis acid sites is that glucose is isomerized to fructose at low temperatures, while direct conversion to industrially important chemicals like alkyl levulinates is viable at higher temperatures.

Acetalisation of bio-glycerol with acetone to produce solketal over sulfonic mesostructured silicas

Sulfonic acid-functionalized mesostructured silicas have demonstrated excellent catalytic behaviour in the acetalisation of glycerol with acetone to yield 2,2-dimethyl-1,3-dioxolane-4-methanol, also known as solketal. This molecule constitutes an excellent compound for the formulation of gasoline, diesel and biodiesel fuels. The activity achieved with arenesulfonic acid-functionalized silica is comparable to that displayed by Amberlyst-15. Optimal production of solketal over arenesulfonic acid mesostructured silica has been established for a reaction system consisting of three consecutive 2-step batches (30 min under reflux and an evaporation step under vacuum), and using a 6/1 acetone/glycerol molar ratio. The use of lower grades of glycerol, such as technical (purity of 91.6 wt%) and crude (85.8 wt%) glycerol, has also provided high conversions of glycerol over sulfonic acid-modified heterogeneous catalysts (84% and 81%, respectively). For refined and technical glycerol the catalysts have been reused, without any regeneration treatment, up to three times, keeping the high initial activity. However, the high sodium content in crude glycerol deactivates the sulfonic acid sites by cation exchange. This deactivation is readily reversed by simple acidification of the catalyst after reaction.

Heterogeneous catalytic wet peroxide oxidation systems for the treatment of an industrial pharmaceutical wastewater.

The aim of this work was to assess the treatment of wastewater coming from a pharmaceutical plant through a continuous heterogeneous catalytic wet peroxide oxidation (CWPO) process using an Fe(2)O(3)/SBA-15 nanocomposite catalyst. This catalyst was preliminary tested in a batch stirred tank reactor (STR), to elucidate the influence of significant parameters on the oxidation system, such as temperature, initial oxidant concentration and initial pH of the reaction medium. In that case, a temperature of 80 degrees C using an initial oxidant concentration corresponding to twice the theoretical stoichiometric amount for complete carbon depletion and initial pH of ca. 3 allow TOC degradation of around 50% after 200 min of contact time. Thereafter, the powder catalyst was extruded with bentonite to prepare pellets that could be used in a fixed bed reactor (FBR). Results in the up-flow FBR indicate that the catalyst shows high activity in terms of TOC mineralization (ca. 60% under steady-state conditions), with an excellent use of the oxidant and high stability of the supported iron species. The activity of the catalyst is kept constant, at least, for 55h of reaction. Furthermore, the BOD(5)/COD ratio is increased from 0.20 to 0.30, whereas the average oxidation stage (AOS) changed from 0.70 to 2.35. These two parameters show a high oxidation degree of organic compounds in the outlet effluent, which enhances its biodegradability, and favours the possibility of a subsequent coupling with a conventional biological treatment.

Bifunctional SO4/ZrO2 catalysts for 5-hydroxymethylfufural (5-HMF) production from glucose

The telescopic conversion of glucose to fructose and then 5-hydroxymethylfurfural (5-HMF), the latter a potential, bio-derived platform chemical feedstock, has been explored over a family of bifunctional sulfated zirconia catalysts possessing tuneable acid–base properties. Characterisation by acid–base titration, XPS, XRD and Raman reveal that submonolayer SO4 coverages offer the ideal balance of basic and Lewis–Bronsted acid sites required to respectively isomerise glucose to fructose, and subsequently dehydrate fructose to 5-HMF. A constant acid site normalised turnover frequency is observed for fructose dehydration to 5-HMF, confirming a common Bronsted acid site is responsible for this transformation.

Etherification of biodiesel-derived glycerol with ethanol for fuel formulation over sulfonic modified catalysts.

The present study is focused on the etherification of biodiesel-derived glycerol with anhydrous ethanol over arenesulfonic acid-functionalized mesostructured silicas to produce ethyl ethers of glycerol that can be used as gasoline or diesel fuel biocomponents. Within the studied range, the best conditions to maximize glycerol conversion and yield towards ethyl-glycerols are: T=200 °C, ethanol/glycerol molar ratio=15/1, and catalyst loading=19 wt%. Under these reaction conditions, 74% glycerol conversion and 42% yield to ethyl ethers have been achieved after 4 h of reaction but with a significant presence of glycerol by-products. In contrast, lower reaction temperatures (T=160 °C) and moderate catalyst loading (14 wt%) in presence of a high ethanol concentration (ethanol/glycerol molar ratio=15/1) are necessary to avoid the formation of glycerol by-products and maximize ethyl-glycerols selectivity. Interestingly, a close catalytic performance to that achieved using high purity glycerol has been obtained with low-grade water-containing glycerol.

Heterogeneous photo-Fenton treatment for the reduction of pharmaceutical contamination in Madrid rivers and ecotoxicological evaluation by a miniaturized fern spores bioassay.

Fifty-six pharmaceuticals of various chemical groups, such as anti-inflammatory, antibacterial and cardiovascular drugs, were detected in four selected river waters receiving sewage effluents in the Community of Madrid (Spain). A promising approach for the degradation of those residues is the application of a photo-Fenton treatment. Several new bioassays using fern spores were employed for the evaluation of acute and chronic toxicity based on mitochondrial activity, DNA and chlorophyll quantifications of as-received river water and photo-Fenton-treated samples. photo-Fenton treatment provided a high degree of total organic carbon mineralization with up to 70% reduction for river water samples. In addition, the elimination of most of the studied pharmaceutical compounds was confirmed. A few compounds, however (salicylic acid, ofloxacin, caffeine, cotinine and nicotine), seemed more resistant, with after-treatment concentrations between 4 and 44ngL(-1). Nicotine showed the most refractory behaviour with concentrations ranging from 29 to 224ngL(-1) for treated samples. Photo-Fenton treatment yielded a significant decrease in acute and chronic toxicity, even though some residual toxicity remained after treatment. This fact seemed to be related to the presence of toxicants in the water matrix, probably of inorganic nature, rather than the toxic effect of the studied pharmaceutical compounds, as revealed by the effective removal of these compounds and high TOC mineralization of photo-Fenton treatments.

Synthesis and catalytic activity of organic–inorganic hybrid Ti-SBA-15 materials

Organic–inorganic hybrid Ti-functionalized mesostructured materials have been synthesized from titanocene dichloride in the presence of non-ionic surfactants as structure directing agents in acidic media. The organic functionalization of the mesoporous silica based materials has been accomplished by different techniques, including direct synthesis in the presence of organosilicon precursors, post-synthetic surface silylation and by using bissilylated organosilane precursors to give titanium-containing periodic mesoporous organosilicas (Ti-PMOs). The presence of bissilylated organosilane precursors in the synthesis medium dramatically influences the textural, structural and morphological properties of the synthesized Ti-PMO materials in comparison with Ti-SBA-15 samples. All these materials show high catalytic activity in the epoxidation of 1-octene with hydroperoxides, their catalytic behaviour being correlated with the hydrophobic nature of the sample.