**1. Introduction**

Non-renewable resources are slowly, but inexorably, depleting [1]. This has prompted researchers to find alternatives to fossil fuel-derived matter, which industry traditionally uses to produce energy and chemicals [2]. Although it is theoretically possible to switch fossil fuels for renewable sources, such as solar, wind, and hydrothermal energy [3], for the post-oil production of chemicals, alternative raw materials are required. Carbon is present on Earth in many forms. It is mostly stored in the lithosphere as carbonate minerals, whereas the part which actively takes part in the carbon cycle is distributed between the soil, atmosphere, and water as carbon dioxide, carbonates, and their intermediates, and as organic carbon in the fossil pool and biomass [4]. Hence, it comes as no surprise that numerous biomass conversion strategies have been developed in recent decades [5]. The extensive use of this strategy may also be effective at reducing the emission of greenhouse gases and tackling the climate change problem [6].

Besides this, renewable energy resources may not always be suitable substitutes for fossil fuels in specific contexts. This may be the case for transportation fuels, for which biomass could instead provide valuable alternatives, namely biomass-derived fuels (or biofuels) [7]. Mixtures must meet some specifications (e.g., viscosity, boiling point, etc.) if they are to serve as diesel or jet fuels [8]; since these fuels are made up of liquid hydrocarbons, the mixtures must contain alkenes with definite carbon chain ranges to meet the aforementioned specifications [9]. The oligomerization of bioethanol [8] and the Fischer–Tropsch reaction of biomass-derived syngas [10] are some of the C–C bond forming strategies

for producing diesel and jet fuels from biomass. Another option is to use bio-based aldehydes as coupling partners in aldol condensations with enolizable ketones such as acetone [11]; the enones obtained after cross-condensation have chain lengths that are suitable for liquid alkane transportation fuels [9]. The total hydrodeoxygenation (HDO) of these products a ffords the corresponding linear hydrocarbons [12]. The enones themselves, and the derivatives obtained by partial HDO, may also be used as novel organic dyes, monomers, and cross-linking agents [13].

A suitable electrophilic partner for this transformation is furfural, a furanic aldehyde obtained by the dehydration of hemicellulose-based pentoses, such as xylose [14]. Furfural is considered to be one of the most important bio-derived chemical platforms, and can be converted into a wide variety of products with the most disparate applications [15]. The reaction of furfural and acetone under basic conditions a ffords the aldol C8-OH (Scheme 1), the dehydration of which leads to the formation of the enone C8. C13-OH is obtained after a second aldol reaction on the other side of the ketone, and C13 is ultimately formed by the dehydration of the corresponding aldol. In industry, aldol condensations are traditionally carried out in alkaline aqueous solution [16], which presents such disadvantages as reaction corrosion and the di fficult separation of the products. For this reason, various reports on the cross-condensation of furfural and acetone have focused on heterogeneous catalysis [17–21].

**Scheme 1.** Base-catalyzed aldol condensation of furfural and acetone.

We developed a microwave-assisted batch process for the neat aldol condensation of furfural and acetone over hydrotalcites and derivatives. Mg:Al hydrotalcites (HTs) are anionic clays which belong to the category of layered double hydroxides with the general formula -*Mg*<sup>2</sup><sup>+</sup> <sup>1</sup>−*<sup>x</sup>Al*3<sup>+</sup> *x* (*HO*−)2 *x*<sup>+</sup>-*CO*2− 3 *x*− *x*/2·*<sup>m</sup>*(*<sup>H</sup>*2*<sup>O</sup>*) [22]. HTs are prepared by co-precipitation of Mg and Al metal salts, and have been used as adsorbents, ion-exchangers, and heterogeneous basic catalysts. In the present study, we prepared 2:1 Mg:Al HTs with di fferent degrees of modification (as-synthesized, calcined, and rehydrated). These solids have di fferent types of basicity [23] and proved to be valuable catalysts for aldol condensation, especially the mixed metal oxides (MMOs) obtained after calcination of HTs [18,24] and the meixnerite-like (MX) layered double hydroxides obtained after rehydration of MMOs [25–27]. The solids prepared were characterized and tested as catalysts in aldol condensation reactions performed in a microwave (MW) reactor. Microwave irradiation has become a popular heating method in organic synthesis [28]. Moreover, as most of the solid catalysts are good MW absorbers, the use of microwave irradiation in heterogeneous catalytic processes is becoming increasingly popular [29]. Neat reaction of furfural and acetone with conventional heating over catalysts similar to ours has already been reported [27], although the hydrotalcites used were purchased, not prepared in the laboratory, and had di fferent properties than the conventional hydrotalcites reported in the literature. We used MW irradiation as a heating method, and used hydrotalcites synthesized in our laboratories.

## **2. Materials and Methods**

All starting materials were obtained from commercial sources. The starting materials for the preparation of the Mg:Al HT were Mg(NO3)2·6H2O (extra pure, Acros Organics, Fair Lawn, NJ, USA), Al(NO3)3·9H2O (≥98%, Fisher Chemical, Hampton, VA, USA), NaOH (Pellets, 98.9%, Fisher Chemical), and Na2CO3 (Anhydrous, ≥99.5%, Fisher Chemical). Deionized water was used for this preparation (18.2 MΩ·cm at 25 ◦C, Simplicity UV water purification system, Merck Millipore, Burlington, NJ, USA). Furfural (99%, Sigma Aldrich, St. Louis, MO, USA) and acetone (99.5+%, Acros Organic) were used in the aldol condensation. Pure N2 (≥99.9992%, 200 bar at 15 ◦C, Carburos Metálicos, Cornellà de Llobregat, Spain) was used to increase the chamber pressure of the microwave reactor during the condensation experiments. Toluene (≥99.3%, Honeywell, Charlotte, NC, USA) was used as an internal standard for the GC-FID analysis of samples obtained after the microwave-assisted process.
