**3. Materials and Methods**

Sulfuric acid (>95%, Fischer, Madrid, España), acetone (99.8%, Fischer), absolute ethanol (Labkem-Labbox, Villasar de Dalt, Barcelona, España), glycerol (99.5%, Sigma-Aldrich, Madrid, España), microcrystalline cellulose (Merck, Kenilworth, NJ, USA) and 1-methyl-naphtalene (96%, Alfa Aesar, Ward Hill, MA, USA) were used as received without further purification. The catalysts Dowex 50Wx2, Amberlyst 15, Nafion-silica SAC-13 and Dowex CCR2 were purchased from Sigma-Aldrich, and Deloxan ASP was purchased form Degussa (Frankfurt, Germany).

### *3.1. Catalysts Synthesis and Characterization*

Glucose sulfonated hydrothermal carbon was prepared in a two-step procedure: hydrothermal synthesis at 195 ◦C and sulfonation with concentrated sulfuric acid at 150 ◦C [30]. The carbon preparation from cellulose follows a similar procedure, in this case HCl can be used to favor the pre-hydrolysis of the raw material [32]. For simplicity, all the solids are noted in the main text indicating their origin (glucose by Glu, or cellulose by Cel) and the preparation conditions such as temperature (195 ◦C or 215 ◦C), concentration of HCl acid (2 M or 5 M) and time of hydrothermal treatment (20 h or 40 h).

In a typical procedure, 2 g of microcrystalline cellulose in 10 mL of pure distilled water (alternatively, 10 mL of either 2 M or 5 M HCl solution were used) were introduced in a 40 mL Teflon-lined autoclave. The autoclave was closed and introduced in an oven at the desired temperature (195 ◦C or 215 ◦C) and it was maintained inside the oven during the desired time (20 h or 40 h). The obtained carbon was filtered o ff and thoroughly washed with water and acetone. Subsequently, the carbons were treated with concentrated (>96%) sulfuric acid (20 mL H2SO4/g solid) in a round-bottom flask furnished with a reflux condenser under argon atmosphere for 15 h at 150 ◦C. The sulfonated samples were then washed thoroughly with hot distilled water and acetone and dried in an oven overnight in static air at 105 ◦C.

A 5 mm-thick graphite felt (GF) sheet was cut into 16 mm diameter circular pieces, which were washed with acetone and treated with nitric acid (65%) in a flask furnished with a reflux condenser with gentle stirring at 80 ◦C for 16 h. The hydrothermal carbon covered graphite felt (HTC@GF) was prepared from GF and glucose or cellulose by hydrothermal synthesis into an autoclave vessel, followed by sonication in water for 10 min and final washing with ethanol to eliminate the hydrothermal carbon loosely bound to the graphite felt. The sulfonation step was carried out under the same conditions used for the HTC, which led to the formation of SHTC@GF. Additional experimental details have been described elsewhere [12].

The obtained solids were characterized by the following techniques. Elemental analysis (C,H,S), were carried out in an Thermofisher Flash 1112 elemental analyzer (Waltham, MA, USA) and CO2 adsorption was carried out for surface area and pore volume distribution determination. CO2 adsorption (Dubinin-Radushkevich) was determined at 0 ◦C using a Micromeritics ASAP 2020 apparatus (Norcross, GA, USA) after outgassing for 4 h at 150 ◦C. For the estimation of the surface area, the Dubinin–Astakhov equation was used.

Scanning electron microscopy (SEM) was carried out with a SEM EDX Hitachi S-3400 N microscope (Tokyo, Japan) with variable pressure up to 270 Pa and with an EDX Röntec XFlash of Si(Li) analyzer (Berlin, Germany). The samples were sputtered with gold previously to measurements and the images were obtained from the secondary electron signal. The mean particle size was determined by measuring 100 particles from images at di fferent locations of the sample.

XPS spectra were recorded with an ESCAPlus Omnicrom system (Taunusstein, Germany) equipped with an Al K radiation source to excite the sample. Calibration of the instrument was done with Ag 3d5/2 line at 368.27 eV. All measurements were performed under UHV, better than 10−10 Torr. Internal referencing of spectrometer energies was made using the dominating C 1s peak of the support at 284.6 eV. The program used to do curve fitting of the spectra was CasaXPS using baseline Shirley method.

Back titration with NaOH was used for the determination of the total amount of acid sites. The solid sample (30 mg) was added to 25 mL of 0.01 M NaOH solution and allowed to equilibrate under stirring for 1 h. Thereafter, it was titrated with 0.05 M potassium hydrogen phthalate solution using a Crison pH Burette 24. The use of potassium hydrogen phthalate (pKa 5.4) avoids the reaction of this acid with the neutralized acid sites on the solid, thus precluding the underestimation of the acid sites, besides it avoids the filtering of the solid before titration that might be sources of errors in the site determination.

Total acid density was defined as the number of total acid sites per m<sup>2</sup> and calculated from titration values and surface area data.

Sulfonic sites density was defined as number of sulfonic sites per m<sup>2</sup> and calculated from sulfur content and surface area data.

Solids were also studied by 13C-CP-MAS-NMR (cross polarization–magic angle spinning–nuclear magnetic resonance). NMR spectra were recorded in a Bruker Avance III WB400 spectrometer (Billerica, MA, USA) with 4 mm zirconia rotors spun at magic angle in N2 at 10 kHz. 1He-13C CP (cross-polarization) spectra (up to 10,000 scans) were measured using a 1H π/2 pulse length of 2.45 μs, with a contact time of 2 μs, and spinal-64 proton decoupling sequence with a pulse length of 4.6 μs.

Triethylphosphine oxide (TEPO) adsorption and 31P-MAS-NMR analysis were also carried out. In a typical procedure, 25 mg of SHTC was suspended in a TEPO methanol solution. The mixture was stirred for 2 h, the solvent was removed by vacuum distillation and the solid TEPO-SHTC sample was analyzed by NMR.

### *3.2. Acetalization Reactions of Glycerol with Acetone Catalyzed by SHTC in a Batch Reactor*

In a typical catalytic test, 4.5 g (0.05 mol) of highly purified glycerol, 25 mL of acetone (0.34 mol, 1 wt% (45 mg) of the catalyst and 0.675 g of 1-methylnaphtalene (GC internal standard), were weighed in a 50 mL glass round bottom flask. The mixture was stirred at 800 rpm at room temperature. The reaction was monitored by gas chromatography by taking samples were taken at di fferent times, which were diluted in methanol and micro-filtrated prior to injection and analyzed in a Agilent 7890 GC with a FID detector and a Zebron inferno column.

In all cases, the liquid reaction mixture was originally biphasic and became monophasic during the reaction.

After the reaction, the catalysts were filtered o ff, washed with methanol and acetone and dried at 105 ◦C overnight before reuse.

In the case of the comparison study of the activity of SHTC with commercial sulfonic solids the amount of catalysts was adjusted depending on the functionalization of the resins Dowex 50Wx2 (4.7 mmol SO3H g<sup>−</sup>1), Amberlyst A15 (4.76 mmol SO3H g<sup>−</sup>1), Nafion-silica SAC-13 (0.16 mmol SO3H g<sup>−</sup>1) and Deloxan ASP (0.8 mmol SO3H g<sup>−</sup>1). Thus in a typical experiment, 0.15 mol of highly purified glycerol, 2 g of 1-methylnaphtalene (GC internal standard), 77 ml of acetone and the amount of catalyst corresponding to a 15% mol ratio of sulfonic sites/mol of glycerol were weighed in a 100 mL glass round bottom flask. The mixture was stirred at 800 rpm at room temperature. The reaction was monitored by gas chromatography by taking samples were taken at di fferent times, which were diluted in methanol and micro-filtrated prior to injection and analyzed in an Agilent 7890 GC with a FID detector and a Zebron inferno column.

In all the cases, TOF was calculated as mol of glycerol reacted per hour and per mol of sulfonic sites, as carboxylic sites were not active in this reaction as it is demonstrated by the lack of activity of carboxylic resin Dowex CCR2 and non-sulfonated HTC (Glu-195-20 h), whereas productivity was calculated as mol of solketal per mol of sulfonic sites.

### *3.3. Acetalization Reaction of Glycerol with Acetone in a Continuous Flow Reactor*

The continuous acetalization of glycerol with acetone was carried out in a flow system, as that schematized in Figure 12. The reactor was an Omnifit ®chromatography column 15 mm in internal diameter and a variable length of up to 100 mm where three SHTC felts were stacked (ca. 75 mg of SHTC). The tubing was 1.6 mm O.D. made of PTFE and connected with luer type connections. The reaction was performed at room temperature and atmospheric pressure. Glycerol and acetone were fed with a "Gemini 88" infusion pump through two independently controlled glass syringes with a total flow from 2 to 17 ml/min. For the case of a single homogeneous liquid phase, glycerol and acetone were previously mixed and 20% *v*/*v* of absolute EtOH was added. A total flow of 5 ml/min was used. The reaction samples were collected each minute in sealed vials, diluted in methanol and analyzed by GC using 1-methylnaphthalene as standard.

**Figure 12.** Experimental set-up for continuous production of solketal in a flow reactor.

The reaction products were analyzed by gas chromatography (GC) using an Agilent 7890 GC with a FID detector and a Zebron inferno column.
