Jose Iglesias

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.

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.

Low-grade oils and fats: Effect of several impurities on biodiesel production over sulfonic acid heterogeneous catalysts

Different lipidic wastes and low-grade oils and fats have been characterized and evaluated as feedstocks for the acid-catalyzed production of FAME. The characterization of these materials has revealed significant contents of free fatty acids, Na, K, Ca, Mg, P, unsaponifiable matter and humidity. Arenesulfonic acid-functionalized SBA-15 silica catalyst has provided yields to FAME close to 80% in the simultaneous esterification-transesterification of the different feedstocks, regardless of their nature and properties, using methanol under the following reaction conditions: 160 °C, 2 h, methanol to oil molar ratio of 30, 8 wt.% catalyst loading, and 2000 rpm stirring rate. Nevertheless, reutilization of the catalyst is compromised by high levels of impurities, especially because of deactivation by strong interaction of unsaponifiable matter with the catalytic sites. The conditioning of these materials by aqueous washing in the presence of cationic-exchange resin Amberlyst-15, followed by a drying step, resulted in a lower deactivation of the catalyst.

Zr-containing hybrid organic-inorganic mesoporous materials:hydrophobic acid catalysts for biodiesel production

Zirconium‐containing periodic mesoporous organosilicas (Zr‐PMOs) with varying framework organic content have been synthesized through a direct synthesis method. These materials display the excellent textural properties of the analogous inorganic solid acid Zr‐SBA‐15 material. However, the substitution of silica by organosilicon species provides a strong hydrophobic character. This substitution leads to meaningful differences in the environment surrounding the zirconium metal sites, leading the modification of the catalytic properties of these materials. Although lower metal incorporation is accomplished in the final materials, leading to a lower population of metal sites, hydrophobisation leads to an impressive beneficial effect on the intrinsic catalytic activity of the zirconium sites in biodiesel production by esterification/transesterification of free fatty acid ‐containing feedstock. Moreover, the catalytic activity of the highly hybridised materials is hardly affected in presence of large amounts of water, confirming their very good water‐tolerance. This makes Zr‐PMO materials interesting catalysts for biodiesel production from highly acidic water‐containing feedstock.

Dehydration of Xylose to Furfural in Alcohol Media in the Presence of Solid Acid Catalysts

Different solid acid catalysts, which include several zeolites, zirconium oxide based catalysts, and materials that contain sulfonic acids, have been evaluated in the dehydration of xylose to furfural in alcohol media. Alcohol media were selected to reduce side reactions, many of which occur in aqueous media. Among the tested alcohols, 2‐propanol provided better results and yielded a higher furfural production than ethanol and methanol. Catalyst screening evidenced that small‐pore‐size zeolites (H‐ZSM5) or catalysts that show weak acidity (tungstated zirconia) were unable to promote the desired transformation. Kinetic studies performed for the different types of materials revealed that the nature of the acid sites influenced the catalytic performance of the different solid acids to the extent of conditioning the main reaction pathway for the transformation of xylose into furfural. Thus, whereas Lewis acids seem to promote furfural production by the direct dehydration of xylose, Brønsted‐type catalysts lead to alkyl xylosides as intermediates in the overall transformation. Although both types of catalysts provide high furfural yields in short reaction times, especially at high temperatures, commercially available β‐zeolite with an adequate combination of Brønsted and Lewis acids sites seems to contain the right physicochemical properties to maximize furfural production.

Maximizing the Accessibility of Active Species in Weakly Acidic Zr‐SBA‐15 Materials

Weakly acidic Zr‐SBA‐15 materials were synthesized by using a novel method to maximize the accessibility of zirconium sites in view of their catalytic applications. Synthesized materials at different ageing temperatures were characterized by means of several techniques such as XRD, N2 physisorption, TEM, ammonia temperature‐programmed desorption, and X‐ray photoelectron spectroscopy and subsequently used in a test acid‐catalyzed process such as the microwave‐assisted alkylation of anisole with benzyl chloride. Reaction results revealed the very high catalytic activity of the prepared Zr‐SBA‐15 materials, which depended mostly on the accessibility of the supported zirconium species and their speciation.

Synthesis and characterisation of (hydroxypropyl)-2-aminomethyl pyridine containing hybrid polymer–silica SBA-15 materials supporting Mo(VI) centres and their use as heterogeneous catalysts for oct-1-ene epoxidation

Hybrid organic–inorganic amino alcohol containing PGMA–SBA-15 materials have been prepared by first tethering poly(glycidyl methacrylate) (PGMA) chains onto the surface of SBA-15 materials using the atom transfer radical polymerization (ATRP) technique. The procedure involves the functionalization of silica-based SBA-15 materials with aminopropyl groups and 2-bromo-2 methylpropionyl bromide to form ATRP initiator species. Subsequent graft ATRP of glycidyl methacrylate leads to a large decrease of the textural properties in the final material, but nevertheless, the use of the ultra large pore SBA-15 support is beneficial for the achievement of porous hybrid organic–inorganic materials. Reaction of the glycidyl pendant groups in the tethered PGMA chains with 2-aminomethyl pyridine allows the formation of the (hydroxypropyl)-2-aminomethyl pyridine ligands to which molybdenum(VI) species catalytically active for epoxidation of terminal alkenes are bound. The materials thus prepared display high catalytic activity and excellent stability and reusability in the epoxidation of 1-octene with TBHP as oxidant. The presence of mesoporosity in the final Mo(VI)-containing hybrid materials boosts the catalytic activity of supported metal centres.

Efficient one-pot production of γ-valerolactone from xylose over Zr-Al-Beta zeolite: rational optimization of catalyst synthesis and reaction conditions

The one-pot conversion of xylose into γ-gammavalerolactone in 2-propanol over bifunctional Zr-Al-Beta zeolites, prepared via a post-synthetic route, was optimized in terms of both catalyst synthesis and reaction conditions. In the catalyst preparation, the use of Zr(NO3)4 as zirconium source as well as the tuning of the amount of water used during the impregnation had a strong impact on the activity of the Zr species due to an improved dispersion of Zr species. As for the aluminium to zirconium exchange, an optimal Al/Zr ratio of 0.20 was identified to provide a catalyst with better activity. The modelization of the catalytic system through experimental design methodology allowed to identify the optimal values of the most influential reaction conditions: temperature 190 °C, catalyst loading 15 g L−1, and starting xylose concentration 30.5 g L−1. Under these optimized reaction conditions, Zr-Al-Beta catalyst provides a GVL yield from xylose (ca. 34%) after only 10 h. The catalysts are stable and reusable after thermal regeneration at 550 °C.

Isosorbide Production from Sorbitol over Heterogeneous Acid Catalysts: Screening and Kinetic Study

The catalytic performance of two types of heterogeneous acid catalysts—sulfonic acid-functionalized materials and aluminum containing zeolites,—in the dehydration of sorbitol to isosorbide, in solventless and autogenous pressure conditions, has been studied. Catalysts screening evidenced strong differences between sulfonic acid-based materials and acid zeolites in terms of catalytic performance. Whereas sulfonic materials, such as Amberlyst-70 and SBA-15-Pr-SO3H, showed a very high catalytic activity, zeolites with beta structure evidenced good catalytic performance together with minimized promotion of side reactions (production of non-desired sorbitans, humins, etc.). Kinetic studies performed at different temperatures, adjusting to a Langmuir–Hinshelwood type model, allowed correlating the physicochemical properties of the acid materials with their catalytic performance in sorbitol dehydration. Thus, the analysis of initial selectivity through kinetic constants comparison indicated that commercial beta zeolite with a Si/Al ratio of 19 is the most selective catalyst for the production of isosorbide, though following a slower kinetics than the sulfonic materials. Furthermore, an equivalent hierarchical beta zeolite has been synthesised and evaluated, resulting in a slight improvement of the catalytic performance, in terms of both yield and selectivity to isosorbide. This improvement is attributed to the superior textural properties.

Biomass catalysis in conventional refineries

Abstract: The feasibility of standard refinery units for the production of biofuels from different biomass-derived feedstocks is highlighted. Special attention is focused on the catalytic cracking and hydrotreating of triglyceride-rich biomass feedstock, probably the most suitable for co-processing in conventional refinery conversion units. Opportunities of other highly oxygenated feedstocks such as pyrolysis oils and sugars are also discussed. Conversion of aqueous sugar streams into conventional liquid fuels by coupling of aqueous-phase reforming (APR) with catalytic systems typical of standard petroleum refineries is evaluated. The chemistry, catalysis and challenges involved in the production of biofuels by the co-feeding of biomass feedstock with petroleum streams are reviewed.