**1. Introduction**

Lignocellulosic biomass is a renewable and accessible feedstock for the production of commodity chemicals and fuels [1]. The main component of raw biomass is cellulose, a biopolymer made from glucose monomers. The hydrothermal treatment of cellulose was first developed to hydrolyze cellulose into liquid fuels or platform chemical molecules such as furfurals [2–5]. The treatment under pressure of cellulose in water, at temperatures in the 150–400 ◦C range, gives rise to a mixture of soluble organic substances and a carbon-rich solid product, whose relative amount depends on the conditions. Only until recently, attention has been paid to the solids resulting from hydrothermal treatment [6–8]. It is widely reported that the hydrothermal treatment of saccharides such as glucose produces carbon microspheres of uniform sizes under very mild process conditions [9–12]. Compared to glucose, cellulose is a more convenient feedstock because it is more abundant and inexpensive and studies can be found in the literature on the hydrothermal carbonization of cellulose [13–16]. Sulfonated hydrothermal carbons derived from renewable raw materials are envisaged as sustainable catalysts and are often applied in biomass transformations [17]. However, the hydrothermal carbons prepared from cellulose have been rarely used as catalyst precursors.

Together with the use of renewable raw materials, the use of green solvents is a trending topic in biorefineries. Among the different alternatives, the use of glycerol itself and its derivatives as renewable

solvents has attracted grea<sup>t</sup> attention in the last decade [18] and, in particular, glycerol carbonate [19] and solketal [20] can be considered the most applied glycerol derived solvents up to now. In order to envisage the possibility of producing large amounts of solketal (Scheme 1), a sustainable synthetic methodology must be developed and, thus, many works have been published so far related to the study of di fferent catalytic systems for the reaction of glycerol with acetone [21]. One of such examples is the use of homogeneous acid catalysts, which implies an aqueous work-up of the reaction. Due to the large transfer of the ketal to the aqueous phase, the global isolated ketal yield is seriously lowered, up to 70% [22]. Thus, many e fforts have been devoted to the search of e fficient heterogeneous catalysts for this reaction. Among the di fferent catalytic systems, heterogeneous Brønsted acids, such as the sulfonic resin Amberlyst 36 [23], sulfonic-functionalized SBA-15 [24] or mesoporous zeolites [25], and heterogeneous Lewis acids, such as Zr- and Hf-TUD-1 and Sn-MCM-41 [26], have been described as effective catalysts for solketal production. In the case of zeolites, the surface area and the presence of mesoporosity seems to also play a crucial role for obtaining high glycerol conversions (80%) and total solketal selectivity. In a recent work, glycidol has been described as starting material for solketal production and several heterogenous catalysts were tried such as Nafion ®NR50, supported metal triflates, K10 montmorillonite, and Amberlyst 15 [27]. In this case, Nafion ®NR50 is described as the best catalyst, although high acetone/glycidol ratios and longer reaction times were needed in order to obtain high glycidol conversions.

**Scheme 1.** Overall reaction for the production of solketal.

It is also worth mentioning that in all the cases, reaction temperatures in between 343–353 K are used in order to achieve good solketal yields.

An interesting point when developing sustainable processes is the possibility of using heterogeneous catalysts that are also derived from renewable raw materials. As mentioned above, this is the case of carbons. Thus, sulfonated activated carbons obtained from olive stones [28] and hydrothermal carbons prepared from glycerine as biodiesel waste [29] have been proposed as catalysts in the reaction of glycerol and acetone. In both cases, the reaction proceeded smoothly at room temperature using 3 wt% catalyst. Conversions of glycerol of 80% and solketal selectivity over 95% were described.

Continuing the e fforts we have done in our laboratory to develop low-cost and versatile biomass derived catalysts, in this work, sulfonated hydrothermal carbons were selected because of their renewable origin, mild preparation conditions and good catalytic performance shown in other acid-catalyzed reactions. More specifically, we present here for the first time the preparation and characterization of sulfonated hydrothermal carbons from cellulose, both bulk and deposited on graphite felt, as well as a comparative study of their activity with a previously described catalyst derived from glucose and commercial sulfonic resins, as acid catalysts in the synthesis of solketal, their reusability and the preliminary results in a flow reactor.
