*2.3. Furfural Distillation*

Furfural spontaneously undergoes degradation when in contact with air [30]. For this reason, furfural was periodically purified, by means of distillation, to remove the degradation products from the starting material. A simple distillation was carried out by heating furfural in a round-bottom flask with a heating mantle (Fibroman-N, JP Selecta, Abrera, Spain). The vapors were channeled into a condenser, and the purified furfural was collected in another round-bottom flask. The flask was sealed, its content was bubbled with argon, and it was stored in a refrigerator at 4 ◦C.

## *2.4. Microwave-Assisted Aldol Condensation*

The aldol condensations were performed in an MW reactor (single reaction chamber microwave synthesis system, Milestone SynthWAVE, controlled by a Terminal 660 control panel with easyCONTROL software, Figure 1a). The reaction mixtures consisted of 0.1 g of furfural (86 uL, 1.04 mmol) in 5 mL of acetone, and the specified amounts of catalyst. The mixtures were placed in oven-dried glass reaction tubes together with magnetic stir bars; caps were put on to prevent the reaction mixtures from coming out of the tubes (Figure 1b). These tubes were placed in a rack, which was immersed in a heating bath. In all cases, this bath consisted of 250 mL of deionized water. The stirring was operated by a rotating magnetic stirrer, and the stirring rate was set to 50%. The whole PTFE reaction vessel was pressurized with 10 bars of N2. The maximum MW irradiation power was set to 300 W. The power was modulated by the system so that the real bath temperature matched the corresponding temperature in the program. Indeed, the temperature probe measures the temperature of the bath, and all temperatures mentioned in the rest of this article refer to the bath ones. The system did not measure the temperature of the reaction mixtures. The general structure of the temperature programs (Figure S1) was the following: in the first step, the temperature was increased to the desired temperature in 10 min; in the second step, this temperature was maintained for the rest of the reaction time; in the third step, the irradiation was stopped, and the bath temperature was allowed to decrease for 20 min to below 56 ◦C, the boiling point of acetone. Once the third step was over, the pressure was released, the reaction vessel was opened, and the reaction tubes were collected.

**Figure 1.** (**a**) Photograph of the microwave (MW) reactor, and (**b**) scheme of the reaction vessel during operation.

#### *2.5. Analysis of the Reaction Mixtures*

Samples were prepared from the final reaction mixtures by adding 100 μL of an internal standard (toluene, which is not formed in the reaction and is not reactive towards any of the reaction components), and then performing 1:5 dilutions with acetone to reduce the concentration of the reaction species and achieve better peak shapes and resolution in the GC analysis. The obtained samples were analyzed with a GC-2010 (Shimadzu, Kyoto, Japan) using a TRB-5 column (TR-120232, Teknokroma, Sant Cugat del Vallès, Spain; length: 30 m; film thickness: 25 μm; inner diameter: 0.25 mm). The product concentration was estimated from the calibration curve obtained for furfural, using the effective carbon number as a correction factor [31]. The conversion *X* of furfural was calculated as follows:

$$X\_{FLR} = \frac{n\_{FLR}^0 - n\_{FLR}}{n\_{FLR}^0} \cdot 100 \tag{1}$$

The yields, *Y*, of the various products and intermediates were calculated as follows:

$$Y\_{\rm C8} = \frac{n\_{\rm C8}}{n\_{\rm FIR}^0} \cdot 100 \qquad Y\_{\rm C8-OH} = \frac{n\_{\rm C8-OH}}{n\_{\rm FIR}^0} \cdot 100 \qquad Y\_{\rm C13} = \frac{2 \cdot n\_{\rm C13}}{n\_{\rm FIR}^0} \cdot 100 \tag{2}$$

The C8:C13 selectivity was defined as the ratio between the yields of the mono- and the double-condensation products. With our protocol, the standard deviations of both conversions and yields are below 5%. In addition, GC-MS was used to aid peak assignations. The samples were analyzed with a GCMS-QP-2010 (Shimadzu) using a Zebron ZB-5ms column (Phenomenex, Torrance, CA, USA). The results of these analyses can be found in the Supplementary Materials (Figure S2).

### **3. Results and Discussion**
