Micro-sized hydrothermal carbon supporting metal oxide nanoparticles as efficient catalyst for mono-dehydration of sugar alcohol

Abstract

Most known catalytic dehydration of sugar alcohols such as D-sorbitol and D-mannitol can only produce di-dehydrated forms as major product, but mono-dehydrated products are also useful chemicals. Moreover, both di- and mono-dehydration demand a high temperature (150°C or higher), which deserves further attentions. To improve the mono-dehydration efficiency, a series of metal-containing hydrothermal carbonaceous materials (HTC) are prepared as catalyst in this work. Characterization reveals that the composition of preparative solution has a key influence on the morphology of HTC. In transformation of D-sorbitol, all HTC catalysts show low conversions in water regardless of temperature, but much better outputs are obtained in ethanol, especially at a higher temperature. When D-mannitol is selected as substrate, moderate to high conversions are obtained in both water and ethanol. On the other hand, high mono-dehydration selectivity is obtained for both sugar alcohols by using all catalysts. The origin of mono-dehydration selectivity and role of carbon component in catalysis are discussed in association with calculations. This study provides an efficient, mild, eco-friendly, and cost-effective system for mono-dehydration of sugar alcohols, which means a lot to development in new detergents or other fine chemicals.

Graphical abstract

Keywords

hydrothermal carbon / morphology / catalyst / mono-dehydration / sugar alcohol

1 Introduction

In view of the gradual depletion of fossil fuels such as coal and oil, worldwide endeavors have been made to use biomass as an alternative energy or industrial raw material [1], of which, the introduction of sugar alcohols including sorbitol and mannitol attracts interests [2]. First of all, these compounds could be produced on a large scale through fermentation and hydrogenation of ubiquitous cellulose, starch or glucose [3]. Besides, sorbitol is the most cost-effective sugar alcohol, taking the largest market share among similar polyols, which is widely employed in the pharmaceutical, food or cosmetic industry [3].

Furthermore, it is significant to see that derivatives of sugar alcohol exhibit great chemical or physical properties for various applications, too [4]. As di-dehydrated forms of sorbitol and mannitol, both isosorbide and isomannide show a great thermal stability, which could be polymerized into engineering plastics [5]. Meanwhile, mono-dehydration of sorbitol and mannitol could provide 1,4-sorbitan and 1,4-mannitan, standing as irreplaceable intermediates for synthesis of detergents (Tween series) [6] and certain pharmaceuticals [7]. Thus, continuous improving the dehydration of sugar alcohol would meet academic, industrial and market needs.

In the early years, mineral acids like hydrofluoric acid [3], sulfuric acid [3], hydrochloric acid [3], or some weak acids [8] were employed as catalyst for this conversion, but these catalysts need additional exothermic neutralization for post reaction treatment. Later, the awareness of safer and cleaner manufacture puts forward solid catalysts for this transformation, including metallic phosphate [9], niobium oxide [10], porous zeolite [11], sulfonic acid-functionalized SBA-15 [12], or superhydrophobic mesoporous acid [13]. Accordingly, dehydration condition is also optimized through the use of microwave heating [14].

To date, there are still three big problems concerning this transformation. First, most of known catalysts have a high acidity, usually facilitating di-dehydration more than the mono-one [9–14]. Secondly, not only di- but also mono-dehydration requires a high temperature for conversion. For example, catalyst-free reaction needs a temperature as high as 317°C, leading to a moderate di-dehydration yield [15]. The use of sulfuric acid needs a temperature of 170°C to guarantee di-dehydration [14], while a temperature of 150°C is the optimized temperature to obtain a moderate di-dehydrated yield with porous Hβ zeolite as the catalyst [16]. The HUSY zeolite [17] and sodium hydrogen sulfate [18] could show mono-dehydrated products as major output, both requiring a temperature higher than 150°C. Lastly, catalyst durability still deserves fundamental optimizations [19]. Obviously, there is still a big room for design and application of new catalysts, which would bring about a new and more powerful reactivity.

Nowadays, fabrication of micro-/nanostructured carbon materials attract interests, like nanodot, nanotube, graphene or derivatives, already finding applications in energy storage, catalysis, separation, sensing or water purification [20]. Some efficient synthetic methods are developed too, including chemical vapor deposition [21], pyrolysis of organic species [22], micromechanical exfoliation [23] or epitaxial growth [24]. Among these methods, hydrothermal carbonization uses sugar or lignin as the starting agent, standing as a convenient, inexpensive and eco-friendly process for producing micro-/nanosized hydrothermal carbonaceous materials (HTC), which show values in water purification [25], adsorption [26,27], and soil enrichment [28]. On the other hand, use of HTC as catalyst or active support deserve attentions. Herein, the texture and morphology of HTC may be modulated by changing preparative condition [29], and meanwhile, undesirable interaction of catalytic component with support could be avoided through use of carbon support [30].

To further stabilize catalyst, coverage of catalyst with graphene oxide (GO) seems to be an attractive option. GO contains several kinds of functional groups, like hydroxyl and epoxy groups on the central part, as well as carbonyl and carboxyl ones on the edge [31], which make its linkage with catalytic component convenient. Then, carbon coverage may have influences on the morphology, stability, electronic structure, and metal center of catalyst [32], which probably means a lot to modulation of catalyst activity.

This study intends to provide an efficient, mild and recyclable system for mono-dehydration of sugar alcohol. In practice, HTC would be prepared through hydrothermal carbonization of D-(-)-fructose in the presence of transitional metal salt. Based on comprehensive characterizations, the relation of catalyst morphology with synthetic condition would be discussed, the factors affecting dehydration efficiency would be probed, and catalytic mechanism is proposed in association with calculations too. This study may provide new insights into utilization of renewable biomass.

2 Experimental

2.1 Materials

The D-(-)-fructose (99%), titanium(IV) n-butoxide (Ti(OBu)₄, 99%), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O, 99+%), cyanuric chloride (99%), bromophenol blue sodium salt (90%), and aluminum oxide (100–200 mesh, neutral, 99%) are purchased from Aladdin. The starting materials used in the synthesis of graphene oxide (GO) including graphite powder (microcrystalline, 325 mesh), phosphorous pentoxide (P₂O₅, 98%), potassium persulfate (K₂S₂O₈, 99%), potassium permanganate (KMnO₄, 99%), and concentrated sulfuric acid (H₂SO₄, 98%) are all commercially available from Adamas. Other reagents like hydrogen peroxide (H₂O₂, 30 wt.% in water), sodium hydroxide (NaOH, 99%), ammonium sulfate ((NH₄)₂SO₄, 99%), silica gel and organic solvents are all analytically pure agents and provided by local suppliers. Distilled water is prepared in the laboratory where the experiment is conducted.

The GO is synthesized through oxidative exfoliation of graphite powder according to a modified Hummers and Offeman method [33]. In practice, the K₂S₂O₈ (5.0 g) and P₂O₅ (5.0 g) are mixed with concentrated H₂SO₄ (30 mL) into a round-bottomed flask (500 mL), whose temperature is slowly raised to 90°C. Then, the microwave-expanded graphite is carefully added to the above acid solution with continuous stirring. The resulting mixture is kept at 80°C for 4.5 h, and then excessive distilled water (250 mL) is added. The fluffy graphite oxide is filtered under reduced pressure, and then carefully washed by distilled water, until the pH value of the filtrate becomes neutral.

The solid is further dried at 60°C in air, and then put into concentrated H₂SO₄ (100 mL) at 0°C. The KMnO₄ (60 g) is introduced slowly with continuous stirring, and the reaction mixture is kept at 35°C with stirring for 2 h. After that, distilled water (250 mL) is added again, and H₂O₂ (30%, 50 mL) is induced slowly, and then the mixture is settled overnight. The precipitates are removed, and the resulting solution is centrifugated and washed with HCl (10%) and distilled water carefully. The final GO solution is a viscous and brownish solution with a concentration of about 35.0% (w/w).

2.2 Instruments

The scanning electron microscopy (SEM) is tested on JEOL JSM-6700F at 20.0 kV without Au coating and the transmission electron microscopy (TEM) is tested on JEOL JEM-200CX at 120 kV. The powder samples are first dispersed into absolute ethanol by ultrasonic bath, and then immobilized onto silicon chip and copper grid for SEM and TEM measurements, respectively. Atomic force microscopy (AFM) is performed on a Veeco Nano Scope IV Multi-Mode AFM system. In practice, the solid sample (0.01 g) is dispersed into absolute ethanol (2.0 mL) by using ultrasonic bath for 20 min, and the resulting mixture is dropped onto a silicon chip (1 cm × 1 cm), which is dried in air, and then tested on AFM. X-ray photoelectron spectroscopy (XPS) is reported on Kratos Axis Ultra DLD, using monochromatic Al Kα X-ray (1486.6 eV) as irradiation source, and binding energy scale is calibrated by using C 1s peak at 284.8 eV. The peaks are fitted by using Gaussian-Lorentz (G/L) product functions with 30% Lorentzian.

Low-angle (2θ = 0.5°–10°) and wide-angle (2θ = 10°–80°) X-ray diffractions are collected on Philips X’Pert Pro diffractometer using Cu-Kα radiation (l = 1.5418 Å), with an interval of 0.05 ^o/s. The static contact angle is measured according to the conventional sessile drop method by using a charge-coupled device (CCD) camera (Sony XC-ST70CE). The suitable sample for measurement of the static contact angle is prepared according to the process for AFM.

The BET surface area, pore volume, pore radius, and pore size distribution are measured on Micromeritics ASAP 2020, using N₂ adsorption isotherms at 77.35 K. The samples are degassed at 150°C in vacuum before testing. The surface area is calculated using the multi-point Brunauer-Emmett-Teller (BET) method based on the adsorption data with relative pressure P/P₀ of 0.06–0.3. The total pore volume is obtained from N₂ adsorbed at P/P₀ = 0.97. The pore volume and pore radius are determined using the Barrett-Joyner-Halenda (BJH) method.

FT-IR is detected in KBr pellets on Bruker Tensor 27, with wave numbers of 400–4000 cm^–1. Thermo-gravimetric analysis (TGA) is performed on NETZSH TG 209C featuring TASC 414/4 controller under nitrogen protection, with a heating rate of 10°C/min at 30°C–800°C. Differential scanning calorimetry (DSC) is conducted on NETZSH DSC 214 under nitrogen protection, with a heating rate of 10°C/min at 30°C–300°C.

GC-MS is tested on GCMS-QP2010 Plus, Shimadzu, with Rxi-5ms capillary column with a length of 30 m and an internal diameter of 0.25 mm. For part of GC, the column temperature is 60°C, the injection port temperature is 250°C, the sampling mode is split-flow, the split-ratio is 26, and the carrier gas is helium. For part of MS, the ion source temperature is 200°C, and the interface temperature is 250°C. UV-Vis spectroscopy is measured on UV 1800, Shimadzu.

2.3 Synthesis of catalysts

As shown in Scheme 1, D-(-)-fructose (0.90 g, 5 mmol), Ti(OBu)₄ (C1, 0 mmol; C2, 5 mmol; C3, 0 mmol; C4, 3 mmol; C5, 1.5 mmol; C6, 2.5 mmol; C7, 2.5 mmol), Al(NO₃)₃·9H₂O (C1, 0 mmol; C2, 0 mmol; C3, 5 mmol; C4, 1.5 mmol; C5, 3 mmol; C6, 2.5 mmol; C7, 2.5 mmol), along with additive (C1–C6, (NH₄)₂SO₄, 5 mmol; C7, NaOH, 5 mmol) are mixed with distilled water (50 mL) into a round-bottomed flask (100 mL) with vigorous stirring. The mixture is then transferred into an autoclave (100 mL) and aged at 200°C for 4 h. The resulting solids are filtered under reduced pressure, washed carefully with distilled water (3 × 5 mL) and absolute ethanol (3 × 5 mL), and then dried in air. The products are all black and caramel-rich powders, including C1 (1.0 g), C2 (1.3 g), C3 (1.4 g), C4 (1.5 g), C5 (1.5 g), C6 (1.5 g), and C7 (1.8 g).

Furthermore, C7 (3.0 g) and GO (3.0 g) are combined with dry toluene (100 mL) into a round-bottomed flask (250 mL) under vigorous stirring. This flask is then being placed in an ultrasound bath for 0.5 h, and a black solution appears. The cyanuric chloride (0.1 g) is added subsequently, and the mixture obtained is further stirred at 80°C for 4 h. The solids are filtered under reduced pressure, washed carefully with distilled water (3 × 5 mL) and absolute ethanol (3 × 5 mL), and dried in air, yielding product C8 (4.5 g).

2.4 Catalytic dehydration of sugar alcohol

The D-sorbitol (or D-mannitol, 1.0 mmol) and catalyst (2 mol% titanium usually, 2 mol% aluminum for C3, 20 mol% carbon for C1, according to the XPS data in Table 1) combining with solvent (20 mL, distilled water or ethanol) are poured into a round-bottomed flask (100 mL) at 20°C or 80°C. The mixture is vigorously stirred for 6 h. Then, the solid catalyst is filtered, and occasionally loaded with consumables for recycling. The filtrate is concentrated under reduced pressure, and then tested on GC-MS for identification and qualification.

2.5 Calculation

The calculations are carried out over Gaussian 09 package [34]. The key front molecular orbitals (MOs) are obtained by theoretical calculations based on their ground states. The functional RB3LYP is selected to optimize the substances, because it is appropriate for discussing reaction profile [35]. Furthermore, all nonmetallic atoms are obtained by using 6–31+ G* group in calculations, and yttrium by using Lanl2dz group.

3 Results and discussion

3.1 Synthesis of catalyst

The synthetic route is shown in Scheme 1, including both the hydrothermal carbonization of fructose and hydrolysis of metal salts. The hydrolysis is performed under either an acidic ((NH₄)₂SO₄) or a basic (NaOH) environment. On the other hand, it was previously reported that 207°C and 82 min appear to be the optimized temperature and time for carbonization of stalks of canola, for the highest mass yield and carbon recovery rate [26]. However, the present work contains hydrolysis of metal salts. For fabrication of more uniform and smaller metal oxide particles in complete hydrolysis, the temperature is decreased to 200°C, while the time is increased to 240 min (Scheme 1). Furthermore, C7 is a covalent linked with GO through cyanuric chloride, an effective double-sided spacer (Scheme 1).

3.2 Characterization of catalyst

3.2.1 Morphology, internal framework and surface details

According to SEM, the preparative solution only provides multilayered blocks in the absence of metal salts (Figs. 1(a) and 1(b), C1 in Scheme 1). When titanium or aluminum is introduced, the product turns out to be uniform microspheres (Figs. 1(c) and 1(d), C2 and C3 in Scheme 1). However, when the molar ratio of titanium with aluminum in preparative solution becomes 2/1 (C4 in Scheme 1), the resulting product contains both microspheres and submicron-sized particles (Fig. 1(e)), probably indicating that the two morphologies have different metal compositions (C4 in Scheme 1).

Furthermore, when the molar ratio of titanium with aluminum is traversed to 1/2 (C5 in Scheme 1), a lot of micro-sized flakes and fibers with a length of several micrometers come into sight (Fig. 1(f)). If the loading amount of titanium and aluminum are equal (C6 in Scheme 1), the resulting product has a similar morphology as C4 (Figs. 1(g) and 1(h)). It seems that the more aluminum is induced, the less microspheres are formed, and the more the flakes, fibers, or submicron-sized particles appear.

When (NH₄)₂SO₄ is replaced with NaOH without changing other preparative conditions (C7 vs. C6 in Scheme 1), the product not only contains microspheres, but also involves fibers having a length of 5–7 mm (Fig. 1(i) vs. Figs. 1(g) and 1(h)). Herein, the use of sodium hydroxide produces an alkaline environment that hydrolyzes metal salt into hydroxide, which is further dehydrated to oxide. Additionally, when C7 is linked with GO, it seems that all subunits of C7 are well covered with a membrane (Figs. 1(k) and 1(l) vs. Fig. 1(i), and Fig. S1, in Electronic Supplementary Material (ESM)), approving that the present linkage is available (C8 in Scheme 1).

The atomic force microscopy (AFM) provides another tool to test the solid surface. First, C3 has a distribution density of about one hundred particles per 2.0 mm× 2.0 mm area, and exhibits particle sizes of 100–200 nm (Figs. S2(a) and S2(b), Section 2 in ESM), corresponding to the submicron-sized particles found by SEM on C3 (Fig. 1(d)). On the other hand, a flake is detected on C5, having a size of 0.4 mm× 0.2 mm (Figs. S2(c) and S2(d)), along with a height of 11 nm (Figs. S2(d) and S2(e)), indicating that C5 contains approximately two-dimensional layers (Fig. 1(f)). The TEM could be employed to test the internal framework. In practice, both C3 and C4 contain solid microspheres, but the solid microspheres of C4 seem larger than those of C3 (Figs. S3(c) and S3(d) vs. Figs. S3(a) and S3(b), in ESM), probably still due to different compositions of the preparative solution (C4 vs. C3 in Scheme 1).

3.2.2 Atomic composition of sample

The XPS measurement is conducted to understand the elemental composition of the sample. The binding energy and atomic composition are summarized in Table 1, and the XPS survey scan is shown in Fig. S4 (Section 4 in ESM). First of all, C2 has a higher content of oxygen, nitrogen and titanium as well as a lower content of carbon and sulfur than C1 (Table 1), indicating that the introduction of titanium excludes carbon and brings about oxygen on the solid product. At the same time, a comparison of C3 with C1 shows almost the same tendency (Table 1), approving the similar effect of aluminum on hydrothermal synthesis (C3 in Scheme 1).

When both titanium and aluminum are induced to synthesis (C4 in Scheme 1), C4 has a higher content of carbon, together with a lower content of oxygen and titanium than C2 (Table 1), but a much larger molar ratio of titanium to aluminum than that in the preparative solution (Ti/Al item in Table 1 vs. that in Scheme 1). When the loading amount of aluminum is increased to be equal to that of titanium (C6 vs. C4 in Scheme 1), the resulting product shows no aluminum (C6 in Table 1).

Furthermore, if the loading amount of aluminum is twice as high as that of titanium (C5 in Scheme 1), there is a small amount of aluminum on the product (Ti/Al= 2.5/1 for C5 in Table 1). Therefore, the incorporation of titanium looks more efficient than that of aluminum under the present condition, which is probably related to the coordination preference of metal ion to fructose during hydrothermal preparation (Scheme 1).

It is interesting to discuss the role of additive in the recruitment of metal ion during hydrothermal treatment. When (NH₄)₂SO₄ is substituted with NaOH (C7 vs. C6 in Scheme 1), both titanium and aluminum are detected on the product (C7 in Table 1), but the incorporation of aluminum looks much better than that of titanium (Ti/Al item for C7 in Table 1), which is different from those found on C4–C6 (Table 1). Therefore, the alkaline solution may have a better effect on improving the precipitation of aluminum salt more than titanium. Additionally, C8 has a higher content of carbon, together with a lower content of oxygen, titanium and aluminum than C7 (Table 1), mainly owing to the coverage of GO (Scheme 1).

3.2.3 Chemical state of element

It is of great interest to further study the effect of the chemical state of element on the sample by using XPS measurement. First of all, C1 exhibits four C 1€s peaks at 284.7, 285.0, 286.1, and 289.0 eV (Fig. S5(a)), which could be ascribed to unsaturated carbon (sp² C), saturated hydrocarbon (sp³ C), carbon of hydroxyl group, as well as that of carboxyl group, respectively [36]. Furthermore, unsaturated carbon accounts for 14.5% of the total carbon of C1 (C= C item, Table S1), probably indicating that there are a considerable amount of turbostratic carbon atoms (sp² C), which may build some staggering six-membered carbon rings on the surface of C1 [28]. The similar phenomena are found on C3–C6 and C8 (Figs. S4(c)–S4(f) and Fig. S4(h), and C= C item in Table 1). However, neither C2 nor C7 show turbostratic carbon as C1 (Fig. S5(b) and S5(g) vs. Fig. S5(a); C (sp²) item in Table 1), suggesting loading titanium as only metal precursor or using NaOH as alkaline additive would depress the turbostratic structure completely.

Next, C2 shows binding energies of Ti 2p_3/2 and 2p_1/2 photoelectrons at 458.7 and 464.3 eV respectively (Fig. 2(a)), and their difference is 5.6 eV, corresponding to the titanium ion of titanium dioxide [37]. All other titanium-containing samples show similar Ti 2p regions as C2 (Figs. 2(b)–2(f) vs. Fig. 2(a)), indicating that there may be only one form of titanium in this system.

On the other hand, C3 shows two contributions on the Al 2p region. The first one that appeared at 74.6 eV could be ascribed to the octahedral aluminum that fixed in a less oxidized environment like alkali salt [38], and the second one at 75.9 eV seems to be the aluminum of metal oxide, having octahedral coordination too (Fig. 3(a)) [39]. Furthermore, both C5 and C7 show similar Al 2p regions as C3 (Figs. 3(b) and 3(c) vs. Fig. 3(a)), approving their close structures in terms of aluminum components.

However, there are two contributions featuring much lower binding energies on the Al 2p region of C8. The first one occurring at 73.4 eV may be attributed to metallic aluminum, while the second at 73.8 eV seems to be the aluminum that incorporated into a tetrahedral coordination, such as alkali salt (Fig. 3(d)) [39], which both appear during the attachment of GO, indicating a reducing procedure (C8 in Scheme 1).

The discussion of the O 1s region would detect sample structure from another point of view. First, C1 shows two contributions at 532.0 and 532.9 eV (Fig. S6(a), Section 6 in ESM), which could be both ascribed to the oxygen of organic species [40]. When metal salt is introduced, the O 1s regions of C2 and C3 become a little sophisticated, where the newly generated contributions that appeared at 530.0 and 530.7 eV are both assigned to the oxygen of metal component (Figs. S6(b) and S6(c) vs. Fig. S6(a)) [40]. In addition, the other five samples show similar O 1s regions as C2 and C3, generally characterizing the oxygens of both metal-containing component and organic specie (Figs. S6(d)–S6(h) vs. Figs. S6(b) and S6(c)).

3.2.4 Crystallinity and hydrophilicity

The morphological and XPS results obtained so far would arouse the interest in detecting sample crystallinity. Both low-angle (2θ = 0.5° –10°) and wide-angle (2θ = 10°–80°) XRD are carried out for powdered samples. Above all, there are no significant diffractions on low-angle ranges of all samples (Figs. S7(a)–S7(h), Section 7 in ESM), indicating that they are lack of mesoscopic symmetry. Then, C1, C6, C7, and C8 show unrecognizable diffractions on wide-angle XRD (Figs. S8(a)–S8(d), Section 8 in ESM), which are indicative of their poor crystallinities.

However, C2 shows typical diffractions of anatase (TiO₂, 2θ = 25.30° (101), 36.94° (103), 37.79° (004), 38.56° (112), 48.03° (200), 53.88° (105), 55.05° (211), 62.10° (213), 62.68° (204), 68.75° (116), 70.28° (220), 75.04° (215); PDF No. 21–1272; gray cube in Fig. 4(a); Fig. S9(a), Section 9 in ESM), indicating that titanium salt is hydrolyzed, precipitated and dehydrated to oxide in the presence of acidic salt (ammonium sulfate) under the present condition (C2 in Scheme 1).

At the same time, the diffractions of NH₄Al₃ (SO₄)₂(OH)₆, as an alkali salt, are detected on wide-angle XRD of C3 (2θ = 14.92° (003), 15.40° (101), 17.66° (012), 25.38° (110), 29.05° (015), 29.57° (113), 29.83° (021), 47.42° (303), 52.14° (220), along with other smaller diffractions; PDF No. 42–1334; dark cube in Fig. 4(b); Fig. S9(b)), indicating that the hydrolysis of aluminum is not complete, and alkali salt appears (C3 in Scheme 1). Additionally, metallic aluminum may be recognized due to the diffractions of 111 (2θ = 38.47°) and 220 (2θ = 65.09°), probably stemming from a pure aluminum phase (PDF No. 65–2869; dot in Fig. 4(b); Fig. S9(b)). Therefore, it seems that aluminum could be reduced to some extent though hydrothermal carbonization (C3 in Scheme 1).

Based on the above identifications, C4 contains three components, including anatase (gray cube in Fig. 4(c) and Fig. S9(c)), NH₄Al₃(SO₄)₂(OH)₆ (dark cube in Fig. 4(c) and Fig. S9(c)), as well as metallic aluminum (dot in Fig. 4(c)), which appear during hydrothermal treatment (C4 in Scheme 1). Furthermore, C5 shows a very similar wide-angle XRD as C4 (Fig. 4(d) vs. Fig. 4(c)), representing their close compositions. On the other hand, taking into account Ti 2p, the O 1s and wide-angle XRD of C4 (Fig. 2(b), Fig. S6(d), Fig. 4(c)), it could be seen that no titanium-aluminum mixed phases come into sight, which is obviously different from those found in silica-based materials [30].

Now that several samples show poor crystallinities, it is reasonable to find their differences through the measurement of the static contact angle. C1 shows a larger static contact angle (average value) than both C2 and C3 (Fig. S10(a) vs. Figs. S10(b) and S10(c) in ESM), indicating pure HTC is more lipophilic than metal-containing ones (C1 vs. C2 and C3 in Scheme 1). C4 shows a larger static contact angle than C2 (Fig. S10(d) vs. Fig. S10(b)), suggesting that the introduction of a small amount of aluminum into hydrothermal carbonization decreases product hydrophilicity (C4 vs. C2 in Scheme 1; C4 vs. C2 for Al item in Table 1). Additionally, C8 shows a lower static contact angle than C7 (Fig. S10(h) vs. Fig. S10(g)), supposing that the attached components including GO and cyanuric chloride are more hydrophilic than C7 (C8 vs. C7 in Scheme 1).

3.2.5 Textural property

The textural property of the synthetic sample could be revealed by using nitrogen physisorption. C1 shows a type II isotherm, an irregular hysteresis loop, as well as a small amount of mesopores (Fig. 5(a)), and meanwhile the BET surface area of C1 is only 3 m²/g (Table 2), indicating that C1 is a non-porous or macroporous material with sparse mesopores [43]. When titanium begins to be loaded (C2 vs. C1 in Scheme 1), the resulting C2 shows type IV isotherm, along with type H3 hysteresis loop (Fig. 5(b)), suggesting a block-like material having silt-shaped mesopores [43]. The rising pore size distribution, higher BET surface area and pore volume all approve this judgment (Fig. 5(b); C2 vs. C1 in Table 1).

Incorporation of titanium decreases the acid amount of sample (C2 vs. C1 in acid amount item in Table 2), probably because the addition of titanium depresses the formation of turbostratic carbon completely (C2 vs. C1 in C= C item in Table 1), which may play the role of an acid center too. In addition to the preparation of C2, the fabrication of C7 also excludes turbostratic carbon, probably due to the alkaline preparative solution (C7 in Scheme 1).

C3 shows a type II isotherm without any hysteresis loop (Fig. 5(c)), the pore size distribution looks considerably marginal (Fig. 5(c)), and meanwhile the BET surface area of C3 is decreased to 2 m²/g (Table 2), all characterizing a non-porous or macroporous material [43]. Overall, in view of the porosities of C1, C2, and C3 (Table 2), the metal-free hydrothermal carbonization only gives dense blocks, but the loading of titanium improves the porosity greatly, while the use of aluminum has little effect (C1 vs. C2 vs. C3 in Scheme 1).

The other four samples including C4–C7 all show type IV isotherms featuring H3 hysteresis loops (Figs. 5(d)– 5(h)), and exhibit BET surface areas of 86–196 m²/g (Table 2), indicating they are block-like materials with silt-like mesopores [43], which prove again that the introduction of titanium would contribute to the construction of the product porosity (C4–C7 in Scheme 1).

When C7 is covered with GO (Scheme 1), the shape of isotherm is basically retained (Fig. 5(h) vs. Fig. 5(g)), while both the BET surface area and pore volume are slightly depressed (C8 vs. C7 in Table 1), which suggests that C7 may have a similar porosity as GO. But at the same time, the micropore volume and acid amount are increased after modification (C8 vs. C7 in Table 1), demonstrating that GO has more micropores and acid centers than C7.

It is available to study component size using various methods. The size of C2 derived from BET surface area is 65 nm (d_S item in Table 2), which is much smaller than microsphere size of C2, being 1.5–1.8 mm according to SEM (Fig. 1(c)). Obviously, the 65 nm describes the non-porous unit of microsphere on C2. Furthermore, the anatase phase size of C2 appears to be 7 nm based on XRD (d_XRD item in Table 2), but no particles having sizes of around 7 nm are found on the SEM of C2 (Fig. 1(c)), indicating that these anatase nanoparticles are incorporated into HTC backbone.

3.2.6 Functional group and thermal stability

FT-IR spectroscopy is conducted in order to further understand the functional group. At first, C1 shows a peak at 2922 cm^–1 (Fig. S11(a), Section 12 in ESM), which could be ascribed to the asymmetric C-H stretching vibration of the methylene group [44]. The bands centered at 1600, 1431, 1273, and 1086 cm⁻¹ could be ascribed to the C-O stretching vibrations of the amide bond [45], carboxyl [46], epoxy [46], and alkoxyl [46] groups, respectively (Fig. S11(a)), which represent the basic components of HTC (C1 in Scheme 1). Furthermore, the bands centered at 789 and 606 cm^–1 could be assigned to the vibrations of SO₄^2– (Fig. S11(a)) [47].

The peak occurred at 3512 cm^–1 on spectrum of C2 seems to be the N-H stretching vibration of NH₄⁺ (Fig. S11(b)). The following peaks at 1608, 1428, 1228, and 1061 cm^–1 characterize the oxygen-containing groups including amide bond, carboxyl, epoxy, and alkoxyl respectively, similar to those recognized on spectrum of C1 (Fig. S11(b) vs. Fig. S11(a)). C2 further shows that the two new peaks that centered at 515 and 415 cm^–1 (Fig. S11(b)) could be attributed to the vibrations of the Ti-O bond [47], indicating that anatase has been incorporated in C2 (Scheme 1). On the other hand, C3, C4 and C5 show very similar FT-IR patterns as C2 (Figs. S11(c)–S11(e) vs. Fig. S11(b)), indicating their close functional groups.

C6 shows a more complex FT-IR spectrum than C2, where the peaks at 3748 and 3635 cm^–1 could be both ascribed to the TiO-H vibrations of the anatase phase (Fig. S11(f)) [47]. The following two peaks at 2980 and 2895 cm^–1 are characteristic of the asymmetric and symmetric stretching vibrations of the methyl group (Fig. S11(f)) [44]. Moreover, C6 shows another three peaks at 1430, 1269, and 1065 cm^–1, corresponding to the C-O stretching of carboxyl, epoxy, and alkoxyl, respectively [46]. In addition, the peaks at 515 and 415 cm^–1 could still be ascribed to the Ti-O stretching of the titanium dioxide phase (Fig. S11(f)).

C7 shows a broad band that centered at 3320 cm^–1 (Fig. S11(g)), probably representing the O-H stretching of the adsorbed water. The following two bands that appeared at 1620 and 1065 cm^–1 are indicative of the O-H binding vibration of the adsorbed water [44] as well as the C-O stretching of the alkoxyl group [46], but other functional groups may be overlapped by the present broad bands (Fig. S11(g)). In the meantime, C8 shows a very similar FT-IR spectrum as C6 more than C7 (Fig. S11(h) vs. Figs. S11 (f) and S11(h) vs. Fig. S11(g)), suggesting GO has very similar functional groups as C6.

It is of significant importance to test thermal stability by using TGA and DSC. Above all, C1 shows a weight loss of 7.6% at 30°C–150°C (black line in Fig. 6(a)), corresponding to an endothermic band appeared at 40°C–150°C, while the small peak at 30°C–40°C looks like instrumental noise (black line in Fig. 6(b)), which could be assigned to the removal of the adsorbed water under heating. There is another large weight loss of 33.02% at 150°C–600°C on TGA (black line in Fig. 6(a)), indicating the departure of the organic species of HTC under heating (C1 in Scheme 1).

C2 shows a more complex TGA curve than C1, which contains three weight losses at 30°C–800°C (red line vs. black line in Fig. 6(a)). The first one of 4.71% appears at 30°C–150°C, also representing the removal of the adsorbed water, which is propelled by a broad endothermic band at 40°C–150°C (red line in Fig. 6(b)). The following one of 11.76% at 200°C–550°C seems to be the departure of the organic species. In addition, C2 shows the last weight loss at 575°C–800°C, probably indicating the removal of involatile organic species. Therefore, it seems that the introduction of metal into synthesis contributes to the formation of involatile species (C2 vs. C1 in Scheme 1).

C3, C4, C6, and C7 show similar TGA patterns as C2, all covering three weight losses (blue, cyan, light brown, and green lines vs. red line in Fig. 6(a)), along with the same kind of endothermic responses at 40°C–150°C (blue, cyan, light brown, and green lines vs. red line in Fig. 6(b)). Meanwhile, both TGA and DSC curves of C5 look like those of C1 (purple line vs. black line, in Figs. 6(a) and 6(b)). On the other hand, the coverage of C7 with GO brings about a similar TGA curve with three weight losses too (brown line in Fig. 6(a)), but in particular gives an additional exothermic band at 150°C–300°C (brown line in Fig. 6(b)), characterizing the elimination of the oxygen-containing groups of GO in the form of oxygen, carbon monoxide, carbon dioxide and water [46]. On the whole, the present HTC samples show a considerable thermal stability below 100°C.

3.3 Catalytic dehydration of sugar alcohol

3.3.1 Effect of temperature

First of all, the optimization of temperature plays a key role in energy saving. When D-sorbitol is used as substrate in water, a higher temperature (80°C) would show a better conversion of D-sorbitol than a lower one (20°C) for aluminum oxide and most catalysts (entries 1–10 and 13–16 in Table S1, Section 13 of ESM), while the only exception comes from the reaction catalyzed by C6 (entries 11 vs. 12 in Table S1). In practice, D-sorbitol is soluble in water at both 80°C and 20°C, a higher temperature may facilitate a more dispersion of substrate into catalytic center than a lower one. But on the other hand, C6 shows the largest static contact angle among all samples (Fig. S10(f) vs. Figs. S10(a)–S10(e), S10(g) and S10(h)), suggesting that the most hydrophobic surface, which may be strengthened by a higher temperature, leads to a poor substrate conversion.

If water is replaced with ethanol as solvent, C2–C5 show higher or equal conversions of D-sorbitol at 20°C than those obtained at 80°C (entries 1 vs. 2, 3 vs. 4, 5 vs. 6, and 7 vs. 8 in Table 3), but C6–C8 do much better at 80°C than 20°C (entries 10 vs. 9, 12 vs. 11, and 14 vs. 13 in Table 3). Overall, C6–C8 show almost equal or much higher pore volumes than C2–C5 (PV item in Table 2), indicating that a higher temperature like 80°C may improve the dispersion of substrate into the internal catalytic centers of C6–C8, leading to a better substrate conversion.

However, the increasing temperature shows little or negative effects on the reactions catalyzed by less porous C2–C5. Herein, C2–C5 show much lower static contact angles than C6–C8 at 20°C (Figs. S10(b)–S10(e) vs. Figs. S10(f)–S10(h)), indicating that C2–C5 have different surface properties than C6–C8, which may show a better affinity to substrate in ethanol at a lower temperature.

When D-mannitol is selected as substrate in water, a higher temperature (80°C) would improve the conversion of substrate more than a lower one (20°C) for transformations facilitated by C2–C4 (entries 16 vs. 15, 18 vs. 17, and 20 vs. 19 in Table 4), while this tendency is traversed for those catalyzed by C5–C8 (entries 21 vs. 22, 23 vs. 24, 25 vs. 26, and 27 vs. 28 in Table 4). This difference may still be ascribed to the affinity of the sample surface to aqueous D-mannitol. Obviously, the interaction of C2–C4 with the substrate might prefer a temperature of 80°C to 20°C, but the use of C5–C8 does better at a lower temperature, which could be also reflected by their different static contact angles (Figs. S10(e)–S10(h) vs. Figs. S10(a)–S10(d)).

But on the other hand, both C6 and C8 show higher conversions of D-mannitol at 80°C instead of 20°C in ethanol (entries 30 vs. 29 and 34 vs. 33 in Table 5), while C7 shows a different result (entry 31 vs. 32 in Table 5). Herein, although C6–C8 exhibit gradual decreasing static contact angles (Figs. S10(f)–S10(h)), C7 contains no turbostratic carbon in comparison with C6 and C8 (C= C item in Table 1), probably affecting its affinity to D-mannitol that dissolved in ethanol at different temperatures.

Lastly, it can be seen that catalysts C6–C8 show high total conversions and high mono-dehydration yields for transformations of D-sorbitol (entries 10, 12, and 14 in Table 3) and D-mannitol (entries 23 and 25 in Table 4; entries 30–32, and 34 in Table 5). Furthermore, these results are obtained under benign conditions including a temperature of 20°C or 80°C, and a solvent of water or ethanol (Tables 3–5). In terms of mono-dehydration efficiency and reaction temperature, the catalytic results of this work are highly comparable to those stemming from H_β zeolite [16], HUSY zeolite [17] and sodium hydrogen sulfate [18], indicating a great application prospect.

3.3.2 Effect of solvent

It is interesting and necessary to discuss the effect of solvent in dehydration, which means a lot to the promotion of conversion efficiency. In general, ethanol seems better than water in catalytic dehydration of D-sorbitol at both 20°C and 80°C (Table 3 vs. Table S1), probably indicating that all catalyst surfaces prefer ethanol-dissolved D-sorbitol to the water-dissolved one no matter how temperature varies.

When D-mannitol is used as substrate, C2–C5 do not show any products in ethanol at both 20°C and 80°C (not shown), but produce detectable dehydrated products in water (entries 15–22 in Table 4). Taking into account that D-mannitol is soluble in both water and ethanol irrespective of temperature, and C2–C5 seem highly non-lipophilic (Figs. S10(b)–S10(e)), the surfaces of C2–C5 may be covered and sealed by D-mannitol that dissolved in ethanol, retarding total conversion.

But at the same time, C6–C8 show moderate to high conversions of D-mannitol in both water (entries 23–28 in Table 4) and ethanol (entries 29–34 in Table 5), probably because C6–C8 are both hydrophilic and lipophilic (Figs. S10(f)–S10(h) vs. Figs. S10(b)–S10(e)), which may accommodate D-mannitol in both water and ethanol.

3.3.3 Effect of catalyst

At first, C1 does not produce any dehydration products under all conditions (not shown), indicating the metal components actually play a key role in dehydration of sugar alcohol (Scheme 1). When the dehydration of D-sorbitol is performed in water, commercial Al₂O₃ shows slightly higher conversions of substrate than C3 at 20°C or 80°C (entries 1 and 2 vs. 5 and 6 in Table S1), indicating that NH₄Al₃(SO₄)₂(OH)₆, the alkaline salt that incorporated in C3 is less efficient than aluminum oxide for this transformation.

C2 shows a better conversion of D-sorbitol than C3 at both 20°C and 80°C in water (entries 3 vs. 5, and 4 vs. 6 in Table S1), and the same trend is also found in ethanol (entries 1 vs. 3, and 2 vs. 4 in Table 3). Herein, although C3 has a higher acid amount than C2 (Table 2), C2 shows a larger BET surface area (Table 2), which may show the major influence on catalytic output.

The conversions of C4 and C5 are considerably close when the dehydration of D-sorbitol is carried out in either water (entries 7 and 8 vs. 9 and 10 in Table S1) or ethanol (entries 5–7 vs. 7 and 8 in Table 3), in spite of temperature. However, when D-mannitol is used as substrate in water, C5 begins to surpass C4 to a large extent at any temperature (entries 21 and 22 vs. 19 and 20 in Table 4). Although C4 has a much larger BET surface area than C5 (Table 2), the dehydration of D-sorbitol may occur more on the surfaces of C4 and C5 than in their internal pore frameworks. But when the substrate is changed to D-mannitol, the dehydration may prefer a higher acid amount of catalyst (C5 vs. C4 in acid amount item in Table 2).

C7 shows higher substrate conversions than C6 in most cases (entries 14 vs. 12 in Table S1; 11 and 12 vs. 9 and 10 in Table 3; 25 and 26 vs. 23 and 24 in Table 4; and 31 vs. 29 in Table 5), probably owing to the higher metal content and acid amount of C7 (Ti and Al items in Table 1; Table 2). Furthermore, the coverage of C7 with GO usually decreases conversions too (entries 14 vs. 12 in Table 3; 27 vs. 25 in Table 4; and 33 and 34 vs. 31 and 32 in Table 5), indicating that the attachment of GO depresses the dispersion of substrate into the metal center. However, C8 shows better conversions of D-sorbitol than C7 at 20°C (entries 16 vs. 14 in Table S1; and 13 vs. 11 in Table 3), which reveals the interaction of D-sorbitol with the metal center would be strengthened through the linkage of GO at a lower temperature.

On the other hand, almost all catalysts produce more mono-dehydrated products than di-dehydrated ones under any temperature or solvent conditions (Tables S1, 3–5, and entry 29 as the only exception), which seems highly comparable to the known catalytic dehydration systems, particularly regarding mono-dehydration efficiency [9–14], temperature [14–18], as well as manipulation convenience [19].

3.3.4 Effect of catalyst recycling and kinetic aspect

It is interesting to test the durability of the synthetic sample in recycling experiment. When C5 is recycled at 80°C in water, the substrate conversion is improved largely at the end of the second use (catalytic cycle 2 in Fig. S12(a), Section 14 in ESM), and then declined gradually (catalytic cycles 3–7 in Fig. S12(a)), probably indicating the degradation of HTC-based catalyst. Nevertheless, there is still an 8% of conversion, after C5 is used for seven times (Fig. S12(a)), approving the durability of C5. In the meantime, C5 always produces mono-dehydrated product as the major one during recycling (Fig. S12(a)), indicating the great mono-dehydration selectivity of this system.

On the other hand, based on UV monitoring (Fig. S13, Section 15 in ESM), it can be seen that the conversions of most reactions would be accomplished within 30 min (Fig. S12(b)), and meanwhile C8 shows a quite different plot of time vs. conversion with C7 (cyan vs. blue in Fig. S12(b)), which could be ascribed to the linkage of GO (Scheme 1).

3.3.5 Proposed mechanism

With the characterization and catalytic data obtained so far, it is reasonable to propose the catalytic mechanism for describing dehydration. At first, the 1,4-dehydration of D-sorbitol could be accomplished by coordination of both 1- and 4-hydroxyl groups with the titanium center of Ti₂O₄ unit, which is a simulation to the anatase phase of the known catalyst (TS1, Part 1 of Scheme 2; Fig. 7(a) and 7(c)). Then, the hydroxyl groups of resulting 1,4-sorbitan would further coordinate to the titanium center for the next dehydration (TS2, Part 1 of Scheme 2).

During this process, TS1 has an energy of –1105.3344 eV, which is lower than that of TS2 (–1028.8888 eV, Fig. 7). Meanwhile, the energy difference between LUMO (lowest unoccupied molecular orbit) and HOMO (highest occupied molecular orbit) for TS1 is 0.12927 eV, while that for TS2 is 0.11958 eV, indicating that TS1 is more stable than TS2 (Figs. 7(a)–7(d)) [48]. Therefore, the mono-dehydration should be the major transformation more than the di-dehydration (entries 3, 4, 7, and 8 in Table S1; and 1, 2, 5, and 6 in Table 3). Similar tendencies could be found among all other transition states (TS3 vs. TS4, TS5 vs. TS6, TS8 vs. TS9, and TS10 vs. TS11 in Scheme 2, Fig. 7, and Fig. S14, Section 16 in ESM), corresponding to the experimental results (Tables S2, 2–4).

To study the role of carbon support in catalysis, a model catalyst containing both titanium and carbon is proposed in TS3 and TS4 (Part 1 in Scheme 2). At first, TS3 shows a lower energy than TS1 (Fig. 7), and LUMO-HOMO gap of TS3 is larger than that of TS1 (Figs. 7(e) and 7(f) vs. Figs. 7(a) and 7(b)), indicating that the presence of carbon component contributes to the stabilization of intermediate, leading to the mono-dehydration (Part 1 in Scheme 2). The comparison of TS4 with TS2 shows a similar variation (Figs. 7(g) and 7(h) vs. Figs. 7(c) and 7(d)), further approving the stabilizing effect of carbon component on di-dehydration (Part 1 in Scheme 2).

At the same time, it is interesting to test the role of carbon support in the reaction catalyzed by aluminum catalyst. In practice, TS7 shows a much lower energy than TS5, and the LUMO-HOMO difference of TS7 is larger too (Fig. 7), obviously showing that the carbon support of aluminum catalyst could improve mono-dehydration efficiency too. When D-sorbitol is replaced with D-mannitol, individual comparisons of TS10 and TS11 with TS8 and TS9 further confirm the introduction of carbon component to catalyst would not only decrease intermediate energy, but also stabilize intermediate during both mono- and di-dehydration procedures (Part 3 in Scheme 2, and TS8–TS11 in Fig. S14).

4 Conclusions

This work prepares a series of hydrothermal carbonaceous materials containing titanium and aluminum components. Characterizations reveal the composition of preparative solution plays a key role in building product morphology. The covalent linkage of one sample with GO is available. In catalytic dehydration of D-sorbitol, all synthetic samples show poor to low conversions in water at both 20°C and 80°C, but the use of ethanol as a solvent increases catalytic outputs greatly. The D-mannitol seems to be a better substrate than D-sorbitol. The coverage of catalyst with GO has positive influences on the transformation of D-sorbitol, but has little influence on that of D-mannitol. Lastly, a catalytic mechanism is proposed to clarify the origin of high mono-dehydration selectivity, as well as positive role of carbon support in catalysis. In general, this study would contribute to the design of carbon-based catalyst and corresponding application for biomass utilization.

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