Microporous humins prepared from sugars and bio-based polymers in

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Microporous humins prepared from sugars and bio-based polymers in concentrated sulfuric acid Fredrik Björnerbäck, and Niklas Hedin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04658 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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ACS Sustainable Chemistry & Engineering

Microporous humins prepared from sugars and bio-based polymers in concentrated sulfuric acid Fredrik Björnerbäck†, Niklas Hedin†,* †Department

of Materials and Environmental Chemistry, Arrhenius laboratory, Stockholm University, SE-106 91 Stockholm,

Sweden * [email protected]

Adsorbents, biomass conversion, carbon dioxide capture, saccharides, glucose, starch, cellulose ABSTRACT: Highly microporous humins were synthesized from readily available sugars and bio-based polymers (monosaccharides, disaccharides and polysaccharides) in sulfuric acid followed by a diethyl ether wash and heat treatment at 400 °C. The relative sustainability, costs of production, and availability of the starting materials were improved significantly as compared with the 5hydroxymethyl furfural-based microporous humins recently studied by us. A multipronged approach was used to study the detailed characteristics of the adsorbents. Results from 1H NMR, 13C NMR, FTIR, WAXS and elemental analysis were combined and showed that the adsorbents predominantly consisted of amorphous and aromatic carbon structures being rich in oxygen. They were highly porous, and the micropore volumes varied among the compositions as could be observed by analyzing CO2 and N2 gas adsorption data. A comparably high CO2 uptake of 4.25 and 1.94 mmol/g at 0 °C and 1 and 0.15 bar was observed. As microporous humins with varying porosities could be synthesized it could expand on the domain of potential applications of this class of materials.

Introduction The non-sustainable use of many natural resources, and its impacts, create a drive towards sustainable industrial processes and materials. Materials with high internal surface areas and large pore volumes are preferred in processes such as molecular separation, purification, catalysis, and energy production and storage,1,2 all of which are likely to play significant roles in an overall more sustainable society. Because of their utility, the sustainability of porous materials themselves is important; however, zeolites,3,4 metal-organic frameworks,5,6 activated carbons,7,8 and porous polymers9 are typically being produced in a non-sustainable manner. Efforts are, hence, being made to enhance their sustainability, in particular for organic porous materials such as activated carbons7,10 and porous polymers.9 Using biomass seems natural to enhance the sustainability of porous materials, and in relation to this, carbohydrates define a broad class of potential precursors with a wide availability. Carbohydrates decompose in acids, such as H2SO4, especially at high temperatures, and form products including 5hydroxymethyl furfural (HMF), levulinic acid, formic acid and precipitated solids/humins.11,12,21–24,13–20 Humins also form in acid hydrolysis of polysaccharides, polymeric carbohydrates, and of cellulose with a suggestive pathway via HMF.25 Humin formation is typically considered a problem as other products are, for good reasons, preferred.26 H2SO4 is an established

reagent and catalyst for the synthesis of certain activated carbons27 and solid acids.28 Porous materials can be produced from simple carbohydrates by hydrothermal carbonization (HTC) in which carbohydrates are chemically dehydrated. From the formed hydrochars, porous carbons can be produced by heat treatments with and without activation agents.20,29–34 Also ionothermal carbonization can be used to form porous carbons from carbohydrates.35–39 Hydrothermal treatment of aqueous solutions of sucrose with acids such as H2SO4 has also been shown to produce hydrochars.40–43 Direct pyrolysis of sucrose produces carbons,44–48 and H2SO4 washing followed by heat treatment can be used to derive porous carbons.47 The direct addition of sucrose to conc. H2SO4 followed by Soxhlet extraction with water for several weeks, pyrolysis, and KOH activation have been shown to produce activated carbons.49 Certainly also complex carbohydrates can be used to synthesize porous materials. For example, cornstarch has been transformed in to a porous material in a multistep procedure including the formation of a hydrogel, drying, heating, and activation with p-toluene sulfonic acid.50 Starch has also been heat treated, sometimes with KOH, to form porous carbons.51,52 Starch nanocrystals can be separated from native starch using acids such as H2SO4.53 Cellulose-based filter papers have been reacted with conc. H2SO4 to produce a porous material.54 Cellulose is known to swell and dissolve in highly concentrated H2SO4, and when subsequently being diluted to form 1

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precipitates that can be used to produce nanocrystalline particles.55 Also hydrochars from complex carbohydrates have been activated into activated carbons.29 In addition, small carbon-rich nanoparticles can be synthesized from organic precursors, such as carbohydrates.56,57 Notably, reacting sucrose with H2SO4 produced a brown solution containing luminous and non-luminous carbon-rich nanoparticles, which could be separated by dialysis.58 Very recently, we showed that aerosols of liquid HMF reacted with H2SO4 could form microporous solids called as microporous humins after a subsequent washing step with diethyl ether.59 As HMF is a quite valuable product, we here present another method that can be used to derive microporous humins by reacting less valuable and more readily available sugars and bio-based polymers using a similar procedure.

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> 0.8 increased only slightly for MHGa while the increase was pronounced for MHSu. The latter N2 sorption isotherm also had a defined adsorption-desorption hysteresis. MHSu, MHL (from lactose) and MHS (from starch) had the most pronounced hystereses. The N2 uptake at p/p0 = 0.99 is used to determine the total pore volume (Vt) and relates to large intraparticlebased pores or interparticle-based voids between particles. The ratios of the micropore volume (Vmic) and Vt were the largest for MHGa and MHF and lowest for MHC. The isotherm for MHC also showed the lowest N2 uptake at low p/p0, and the highest N2 uptake at high p/p0, which indicated that this sample contained comparably large pores. Kinetically, these large pores can be advantageous in potential gas or liquid based separation processes as the mass flows could be increased.

Results and discussion Microporous humins were produced from a range of different sugars or sugar-based molecules by reactions in conc. H2SO4 at a temperature of 50 °C. A work up procedure with diethyl ether washing and a subsequent heat treatment to 400 °C improved on the porosities of the formed microporous humins. Numerous different reactions were performed with glucose (MHG), fructose (MHF), galactose (MHGa), sucrose (MHSu), lactose (MHL), starch (MHS), and cellulose (MHC) as reactants. MH abbreviates microporous humins. A detailed discussion of the optimization of the different steps of the synthesis, with respect to the yield and CO2 uptake, is presented in the Supporting Information together with some findings of significant color alternations in some of the preparations. Textural properties of microporous humins The character of the porosity of the microporous humins were studied by gas adsorption techniques, and the textural properties and CO2 uptake levels are presented in Table 1. Overall, the microporous humins had similar textural properties and high CO2 uptake, irrespectively of the staring material used. The N2 adsorption and desorption isotherms were similar at the low relative pressures (p/p0) corresponding to the micropores (see Figure 1). At higher p/p0, the shapes were different, as could be exemplified by comparing the N2 sorption isotherms for MHSu (from sucrose) and MHGa (from galactose). MHGa has a N2 adsorption isotherm shape similar to Type Ib, and MHSu an isotherm similar to type II.60 The N2 uptake for p/p0

Figure 1. N2 adsorption/desorption isotherms collected at -196 °C for MHG (blue), MHF (green), MHGa (yellow), MHSu (orange), MHL (grey), MHS (dark purple) and MHC (pink). Closed squares and open squares correspond to adsorption and desorption isotherms.

The microporous humins had large BET specific surface areas (SBET), Vt , and Vmic when being compared to materials prepared under relatively similar conditions as can be seen by the comparison in Table 2 and Table S8. Microporous activated carbons (ACs) can have much higher Vt or Vmic, (Table S815,61,70– 75,62–69), but are chemically different materials. The V of the t microporous humins of this study was somewhat lower than those produced with an aerosol method from HMF59 in conc. H2SO4; however, that procedure was more complex, and HMF is a valuable chemical. Table 1. Textural properties and CO2 uptake for microporous humins. Sample

SBET (m2/g)

SEXT (m2/g)

Smic (m2/g)

Vt (cm3/g)

Vmic (cm3/g)

Ratio Vmic:Vt

MHG MHF MHGa MHSu MHL MHS MHC

811 772 762 828 753 834 740

53 43 44 64 57 66 76

758 729 718 764 696 768 699

0.37 0.33 0.32 0.40 0.35 0.40 0.44

0.30 0.29 0.28 0.30 0.28 0.31 0.27

0.81 0.89 0.88 0.75 0.79 0.78 0.62


CO2 uptake, 1 bar, 0 °C (mmol/g) 4.25 4.10 4.06 4.15 3.85 4.11 3.78

CO2 uptake, 0.15 bar, 0 °C (mmol/g) 1.94 1.90 1.89 1.86 1.75 1.78 1.75

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Table 2. Textural properties for a range of selected organic sorbents synthesized at low temperature (≤ 400 °C). Starting material, synthesis

SBET Vt (m2/g) (cm3/g) Previous, similar, work 5-hydroxymethyl furfural, H2SO4 1020 0.73 Saccharides Fructose, ionothermal 533 0.38 a Filter paper, H2SO4 377 a Glucose, HTC and H2SO4 357 Other Coffee fruit husk, FeCl3, and ZnCl2 1374 0.65 a Common vetch, ZnCl2 1287 a Silver berry seeds, ZnCl2 697 Coal, H3PO4 458 0.24 Bagasse, H2SO4 403 0.013 Cocoa shell, ZnCl2 and HCl 288 0.16 Coconut shell, only N2 130 0.19 a = not reported. V was estimated from N isotherms. t 2

isotherms recorded at 0, 10 and 20 °C and the CO2 adsorption isosteres are presented in Figures S2 and S3.)

Ref. 59

35 54 20

76 77 78 79 80 81 82

Very small pores are often studied with adsorption of CO2 at 0 °C,83 and the general shape of such adsorption isotherms are typically similar to those of the microporous humins, as displayed in Figure S1. Slight adsorptiondesorption hystereses were observed, which may have been related to micropores with narrow pore throats, transformation of the (ad)sorbent structure, chemical or physical aspects of the (ad)sorbate-(ad)sorbent interactions, pore blocking or kinetic trapping.84–86 The initial slopes of the isotherms in Figure S1, with high CO2 adsorption at low pressures, were consistent with pore systems having significant amounts of ultramicropores and in agreement with the shape of the N2 adsorption isotherms (c.f. Figure 1). The level of CO2 adsorption was comparably high on the microporous humins, which is crucial to their potential use as adsorbents for capture of CO2 from flue gas.87 The uptake at low pressures is relevant for CO2 separation processes in general as the partial pressure of CO2 is typically low.75 At 0.15 bar and 0 °C, the CO2 adsorption of the microporous humins were comparably high.88 The SBET and CO2 uptake levels (at 1 bar and 0 °C) were comparable with typical microporous polymers in general.89 The CO2 uptake of the microporous humins were higher than for most activated carbons in the pressure regime of 0–1 bar at a temperature of 0 °C and compares well with those of many zeolites.90 Certain porous polymers, activated carbons and zeolites have much higher CO2 uptakes (see Table 3) but the microporous carbons have potential advantages related to the relatively nonpolar surfaces and sustainable nature of the synthesis. The loading-dependent isosteric heat of CO2 adsorption (Qst), or binding energy of CO2 to the internal surfaces, was determined for the MHG (from glucose) and presented in Figure 2. At low adsorbed quantities, the Qst was 34 kJ/mol and decreased gradually to 29 kJ/mol as the adsorbed quantity increased. These values were comparable to those of microporous polymers,89 and reduction in Qst (5 kJ/mol) on loading indicated surface inhomogeneities91 or poresize variations. (The corresponding CO2 adsorption

Figure 2. Heat of CO2 adsorption for the microporous humin MHG (prepared from glucose).

Table 3. Carbon dioxide uptake data for selected sorbents. CO2 uptake, CO2 uptake, 1 bar, 0 °C 0.15 bar, 0 °C (mmol/g) (mmol/g) Previous, similar, work MHH-4 5.27 2.18 Organic porous polymers HMC-3 7a HAT-CTF-450/600 6.3 3.0 PPF-1 6.07 2.2a Activated carbons Petroleum pitch 8.6 1.7a Pine-nut shell 7.7 3.3 HTC Algae 7.4 MAC-E-7 6.0 1.8 Other Zeolite 13X 4.1 3a Sorbent/starting material

Ref. 59

92 93 94

95 96 97 8

98

Molecular structure of microporous humins The microporous humins contained 67.2–75.3 wt. % of carbon in the order of MHG < MHGa < MHSu < MHL < MHS < MHF < MHC, which was inversely related to the O/C atomic ratios (oxygen content is calculated by subtraction) as can be seen from the positions in the van Krevelen diagram99 of Figure 3. The H/C atomic ratios of the microporous humins varied only little. The positions of the microporous humins and the starting materials in the van Krevelen diagram indicated that dehydration reactions had occurred and were consistent with that aromatic carbon structures had formed. Dehydrogenation reactions had likely also occurred as the relation between O/C and H/C ratios placed the microporous humins below the diagonal of Figure 3, which would be consistent with the formation of C-C bonds. The positions below the diagonal could also be interpreted as positions to the right of the diagonal, because of an increased oxygen content which, tentatively, could have been caused by oxidative reactions by sulfuric acid. The off diagonal position could relate to dehydrogenation or oxidation reactions. By evaluating the elemental compositions of the microporous humins as a function of 3



the type of starting material, i.e. monomer, dimer or polymer, did not reveal any clear pattern. The sulfur content was higher than for other microporous humins synthesized under similar conditions59 and was likely introduced through the use of conc. H2SO4. However, no sulfur content was detected for MHC. At the moment, we have no explanation to the variation in the S-content. The CO2 uptake and the CO2 uptake normalized to micropore surface area for the microporous humins were investigated with respect to the elemental composition and correlation plots are presented in Figures S4-S11. The CO2 uptake was somewhat negatively correlated with the carbon content of the microporous humins. (The elemental analysis was not conducted with pretreatment to remove adsorbed components, and the related data (Table S7, and Figure 3) may include the content of adsorbed water).

Figure 3. van Krevelen diagram for microporous humins and their starting materials.

We used solid state NMR and infrared spectroscopy to reveal that the microporous humins were molecular in their structural nature and not traditionally carbon-like, irrespectively of their positions in the van Krevelen diagram of Figure 3. MHC, MHG, and MHS were selected for the NMR analyses as representatives of two different types of microporous humins having comparable IR spectra. MHC had an IR spectrum similar to MHF whereas MHS and MHG had similar spectra to those of MHL, MHSu and MHGa. Solid state cross-polarization (CP) {1H} 13C NMR spectra were recorded under conditions of Magic Angle Spinning (MAS) and are presented in Figure 4.

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Figure 4. Solid state {1H}13C NMR spectra, using CP, and MAS (14 kHz), for selected microporous humins (* marks the spinning sidebands). (The spectra for MHG were recorded at a lower field than the others).

Broad bands centered at 13C NMR chemical shifts of 126 and 155 ppm are identifiable in the spectra of Figure 4. The chemical shifts for these peaks are typical for aromatic sp2 carbon and for phenols or linked –HC=C-O in furanic rings. The shoulder at roughly 110 ppm can be related to phenols or linked –HC=C-O in furanic rings. Note that a signal from a carbonyl groups was observed only for the MHG recorded at a frequency of 100.6 but not of 150.9 MHz. This 13C chemical shift was observed at roughly 185 ppm. This difference was tentatively assigned to differential tendencies to the cross-polarization dynamics at different magnetic fields. When comparing the spinning sidebands of MHC, there are significantly larger sidebands on the low-ppm side of the central bands, which we assign to an additional contribution from aliphatic carbons. For MHS and MHG, peaks from aliphatic carbons were not clearly observed. Adsorbed water affected the 1H NMR spectra, see the supporting information. The IR spectra of the microporous humins were normalized to the band intensities at a wavenumber of around 1590 cm-1 Figure 5) and only minor differences were observed in the so-called fingerprint region (1500– 500 cm-1). This region was similar for MHC and MHF, and for MHS, MHL, MHSu, MHGa, and MHG. The distorted band shapes in the spectra indicate that they were not in a pure absorption form, which was likely due to reflectancerelated phenomena.100,101 This distortion made the peak assignments difficult to perform and contributed to associated uncertainties in the peak positions and intensities. The band at a wavenumber about 1590 cm-1, observed for all samples, was assigned to the C-C stretching vibrations of in aromatic rings102 and consistent with the NMR assignments. This band shifted slightly from 1583 to 1597 cm-1 for MHGa = MGS = MHG < MHL < MHSu < MHF < MHC (low to high wavenumbers). The band about 1710 cm-1 was assigned to the C=O stretching vibration103 and shifted slightly in the order of MHG = MHGa = MHSu = MHS < MHF = MHL (low to high wavenumbers). MHC had a higher wavenumber of about 1715 cm-1. Note, the relatively low concentration of carbonyl groups made it difficult to detect these groups in the {1H}13C NMR spectra. The broad feature observed in every spectrum at frequencies >2700 cm-1 is typical for OH stretching vibrations and likely related to hydroxyl groups, or adsorbed water.102

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Figure 5. Infrared absorbance spectra for MHG, MHF, MHGa, MHSu, MHL, MHS and MHC (bottom to top).

X-ray scattering aspects and thermal stability of microporous humins Scattering or diffraction of X-rays in the wide angle domain can be used to study aspects of microporous polymers104,105 and porous carbons.37,106 Graphitized carbons generally show peaks at roughly 26, 43, 54 and 65– 78° (2θ) which corresponds to the 002, 100, 004 and 110 reflections. The 002 and 004 reflections correspond to the separation distances of aromatic layers, the 100 reflection corresponds to the second-neighbor distance in the aromatic rings, and the 110 reflection corresponds to the first-neighbor distance in the aromatic rings.106–109 The Wide Angle X-ray Scattering (WAXS) curves for MHG, MHS and MHC (Figure 6a) showed characteristics typical of X-ray amorphous (non-crystalline) materials with broad peaks. A shoulder at 10° (2θ), and broad peaks at 20–22, 43, and 80° (2θ) were detected for all microporous humins. Evaluating the scattering intensity as a function of the distances d (Figure 6b) revealed four distinct but broadly distributed d of roughly 1, 0.40–0.45, 0.21, and 0.12 nm. The sharp peak at 25.4° (2θ) for MHS and tentatively also for MHG corresponded to d = 0.35 nm.

Figure 6. (a) Wide angle X-ray scattering curves for MHG, MHS and MHC, and (b) the scattering intensity as a function of the distances.

The shoulder centered at 10° (2θ), or d=1 nm, have been related to oxygenated functional groups in, or water molecules between, layers of aromatic carbons.110,111 There is a large amount of oxygen in the microporous humins (roughly 22–28 wt. %, see Table S7), and IR (Figure 5) and NMR (Figure 4) spectroscopy data point towards water, hydroxyl and carbonyl functionalities. We have no clear indication of pore sizes of d=1 nm by gas sorption analysis (Figure 1 and S1). CO2 or N2 gas molecules certainly would have had access to (open) pores in that size range. Tentative density functional theory analyses (slit-shape, carbon, Figure S12) of the N2 adsorption data indicated that the microporous humins contained pores with sizes of about 0.8-0.9 nm and only few pores ≥ 1 nm where significant features were observed in the WAXS curves. The shoulder around d=1 nm was assigned to structural carbon features and, not specifically to accessible pores, to achieve consistency between gas adsorption and WAXS analyzes. The shoulder around d = 1 nm was most pronounced for MHG as can be most clearly seen in the raw data (Figure S14), which could have been related to the high calculated oxygen content of MHG. MHG did not, however, show the largest mass loss of adsorbed water as was determined from the loss at 150 °C in air (Figure 8, discussed below). The broad peaks centered at 20° for MHC, 21° for MHG and 22° (2θ) for MHS corresponded to d of 0.44, 0.42 and 0.40 nm were the most dominant features in the scattering 5



curves of the microporous humins. They are typically assigned to the chain-chain distances for densely packed polymers.105 The most probable explanation for these scattering features seems to be network packing distances of a non-porous nature. We could inversely relate the rank order of the d values to the mass loss of adsorbed water (Figure 7), Vmic, and SBET (Table 1) but not to any other aspect of the gas sorption analyses or molecular analyses (NMR, IR, and elemental analysis). However, we do not speculate about the microscopic reasons of these correlations. The sharp peaks observed at 25.4° (2θ) and the broad peaks at 43° (2θ) matched well with those for graphitized carbon, corresponding to the separation distance between aromatic layers (d = 0.35 nm) and the distance between planes of second-neighbors in the aromatic rings (d = 0.21 nm). The sharp peak at 25.4° (2θ) may have been related to a higher degree of interlayer ordering of the aromatic structures. The 110 reflection, corresponding to the distance between planes of firstneighbors in aromatic rings, are typically seen at 6578° (2θ). For microporous humins, this peak was centered at 80° (2θ) with a d = 0.12 nm, which (assuming regular carbon hexagons) corresponded to carbon bond lengths distributed around 0.139 nm, being in the range of the typical values of 0.133–0.142 nm.108,109 Background WAXS data were subtracted from the normalized sample data. Isopropanol was used in the preparation of the samples for WAXS analysis. IR and NMR spectroscopy and elemental analysis (not presented) showed samples to contain isopropanol days after the WAXS analysis. We speculate that adsorbed isopropanol may have affected the WAXS data. Isopropanol could be removed by heating under a flow of N2. Thermogravimetric analyses (TGA) of MHG, MHS and MHC (Figure 7b) revealed that the microporous humins were stable up to 300 °C in air. At temperature of 150 °C, the losses were 4.1, 4.3 and 3.3 wt. % for MHG, MHS and MHC. Isothermal flow of dry air for 60 minute at ambient temperature caused mass losses of (2.8, 1.7 and 2.2 wt. %), Figure 7a. These losses were tentatively assigned to adsorbed water. For MHG, a minimum value of the mass was observed with 1.5 wt. % at 480 °C, unexpectedly, the mass increased up to 3.1 wt.% at 600 °C. For MHS, 1.1 wt.% remained at 460 °C, and no mass of MHC was detected above 470 °C. The thermogravimetric responses for the different microporous humins were similar but had some differences in the slopes of the curves.

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Figure 7. Thermogravimetric analysis of MHG, MHS and MHC; (a) Isothermal in a flow of dry air for 60 minutes, and (b) with a subsequent heating rate of 1 °C/min (b).

Scanning electron microscopy With the SEM images of Figure 8, and Figure S15-21, we observed that all the microporous humins consisted of interconnected particles and that channels or pores had formed between the particles. The interconnections led us to speculate if these microporous humins could be advantageous to application in which mass flow restrictions need to be minimal. Similar morphologies have been observed in a related study,59 and other carbonaceous solids.112–117 From Figure 8a and 8c it seemed that the particles were least fused for MHG and MHC. The particles of MHGa (Figure S17) on the other hand seemed to have been more fused together than the other microporous humins. Parts of the sample MHSu also seemed to consist of significant amount of fused particles (Figure S18). MHS and MHC (Figures 8b and 8c) had the smallest particle sizes. MHF (Figure S16) had the largest particles. Even if MHF and MHC appeared to be chemically very similar, as judged from IR spectra, their particle sizes were significantly different.

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ACS Sustainable Chemistry & Engineering experiments. The gentle-beam technology was used in combination with a small probe current to reduce the effects of charging.

a

Conclusions Microporous humins were prepared from monosaccharides, disaccharides and polysaccharides in conc. H2SO4 with subsequent washing in diethyl ether and heating in a flow of N2. The aerosol-based method that had used for HMF in another study was found less applicable to carbohydrates dissolved in water. The fraction of micropores varied among microporous humins, and they had comparably high uptake of CO2, large surface areas and pore volumes. They had amorphous and aromatic carbon structures with varying amounts of O atoms. Variations in the pore structures and the surface chemistry of these humins indicate that they have potential for further tailoring for specific applications. In addition, they also typically contained a small amount of sulfur. Further expanding the class of microporous humins class should be possible by using other saccharides, furfural derivatives and related molecules, as they are likely to share similar reaction pathways. The precipitation effect of added diethyl ether could be further studied, and we expect that by using other solvents interesting results could emerge. The observation of carbon-rich nanoparticles or other colored intermediates in the synthesis of the microporous humins were clearly interesting and could inspire further studies, both in relation to the very mechanisms of formation and potential applications.

b

Experimental

c

Synthesis

Figure 8. Scanning microscopy images of MHG (a), MHS (b) and MHC (c). The magnification was 60,000x.

The seemingly non-fused nature and small particles of MHC could explain its low Vmic:Vt ratio. The small particles could have led to significant mesopore volumes relating to the interparticle voids. The more fused particles of MHGa may explain its high Vmic:Vt ratio and associated reduced mesopore volume. The microporous humins were also SEM imaged at lower magnification and related images are presented in Figure S15-S21. It seemed as the conductivity of the microporous humins was low, as strong charging had occurred during the SEM

D-(+)-glucose (>99.5%) and D-(-)-fructose (>99%) were supplied via Sigma-Aldrich. D-(+)-galactose (98%), sucrose (99%), D-lactose, and starch (from potato, soluble) were supplied via Alfa Aesar. Cellulose, in the form of grade-3 filter paper was supplied via Munktell Ahlstrom. 95% H2SO4 and diethyl ether were purchased from VWR. Glucose, fructose, galactose, sucrose, lactose, starch, and cellulose were used as starting materials for the syntheses. In a typical procedure, 10 ml of conc. H2SO4 was heated to 50 °C in a centrifuge tube, and 0.63 g of glucose was added to the tube while stirring. Caution: conc. H2SO4 should be handled with the proper precautions and care. The tube was shaken for 20 s, centrifuged briefly, put back on heating to 50 °C and kept open at this temperature for 24 h. The tube was subsequently put into a water bath for cooling and diethyl ether was added with care to a total volume of 50 ml while stirring (care should here be taken because of boiling upon addition of diethyl ether). The mixture was centrifuged and the liquid phase removed. This procedure was repeated for a total of three times. The solid sample was collected in a glass tube, dried, and heated to 400 °C in a flow of N2. Variations of this procedure and key observations are presented in Tables S1-S6. For the heat treatment >400 °C, a tube furnace with N2 flow was

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used. Yields were calculated by dividing the sample mass after heat treatment with that of the starting material. Characterization Samples were initially screened with respect to the porosity by recording the CO2 adsorption isotherms with a Micromeritics Gemini VII device. Prior to analyses, the samples were subjected to a flow of N2 gas for at least 8 hours at room temperature and then degassed (in a flow of N2) at 400 °C overnight (>12 hours), and subsequently allowed to cool down in the flow of N2 gas to room temperature. A Micromeritics ASAP 2020 device was used to record the adsorption and desorption isotherms at gas pressures of 0–1 bar. Before analyses, the samples were degassed in two steps. This included heating to 50 °C at a rate of 10 °C/min with evacuation until a pressure of maximum 10 μmHg was reached for 30 min, after which the samples were heated to 400 °C, at a rate of 2 °C/min and kept under dynamic vacuum for > 5 h. The sample tubes were allowed to cool down and backfilled with N2 gas. N2 isotherms were recorded at -196 °C, and CO2 isotherms at 0 °C. For the calculation of the heat of adsorption of CO2, additional CO2 isotherms were recorded at 10 and 20 °C. The SBET was calculated, using the Brunauer-Emmet-Teller118 model using the N2 adsorption data recorded at -196 °C at p/p0 of 0–0.041 for all samples. In this pressure range, the n(1p/p0) increased with the p/p0, and the parameter c was positive. The external surface area (SEXT) and Vmic were determined by using the t-plot method for p/p0=0.2–0.5 using the method for carbon black (ASTM D6556-10).66 The micropore surface area (Smic) was calculated as the difference between SBET and SEXT. The Vtot was calculated according to the relation Vtot = VN2/Vmol*MN2/ρN2 (1) at p/p0 = 0.99, and VN2 is the volume (cm3) of N2 uptake per gram of material, Vmol is the volume of one mole of gas in STP (22414 cm3), MN2 is the molecular weight of the N2 molecule (28.01 g/mol) and ρN2 the density of liquid N2 (0.807 cm3/g). SEM images were recorded with a JEOL JSM-7401F instrument, equipped with an in-lens secondary-electron column detector. A gentle beam methodology was applied with an accelerating voltage of 3 kV, a negative bias of 2 kV on the sample stage, and a landing energy corresponding to 1 kV. Working distances of 1.4–1.7 mm were used and the samples were fixated with an ink. TGA was performed using a TA Instruments Discovery TG device up to a temperature of 600 °C in a flow of dry air with a heating rate of 1 °C/min. Before heating, the samples were placed in the flow of dry air for 60 minutes and the loss in weight was recorded also for this period. WAXS curves were recorded on a PANalytical X’pert alpha1 powder diffractometer with a PIXcel detector (CuKα1 radiation, k = 1.5406 Å) using a reflection mode with 2θ = 3.0° – 90°. The values of the interlayer distance were calculated according to d = k/(2*sin(θ)). Samples were ground, mixed with ethanol and dried on a silicon crystal sample plate prior to measurements. IR spectra were recorded on a Varian 670-IR spectrometer with a Specac Golden-gate attenuated total

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reflection device using a room temperature detector based on deuterated triglycine sulfate. (The spectra were not adjusted for the deeper penetration towards lower wavelengths.) The elemental compositions of the samples were determined by using combustion analysis. A subtraction was used to estimate the mass of O under the assumption that samples consisted entirely of C, H, N, O and S. Solid state 13C and 1H NMR spectra were recorded under MAS using 600 MHz and 400 MHz Bruker AVANCE III spectrometers and 4-mm probe heads with 13C frequencies of 150.9 and 100.6 MHz. The MAS spinning rate was 14 kHz. For the {1H}13C NMR experiments, SPINAL decoupling of the 1H contributions were used and the 9– 32 k 13C transients were recorded. A ramped CP pulse of 1 ms was used to transfer magnetization from 1H to 13C. A line broading of 500 Hz was applied to the 13C NMR spectra, and the 1H and 13C chemical shifts were externally calibrated with those of adamantane.

ASSOCIATED CONTENT Supporting Information. Synthesis details (Tables S1-S6), Elemental compositions and textural properties (Table S7-S8), CO2 isotherms (Figure S1-S2), CO2 isosteres (Figure S3), CO2 uptake correlations to elemental compositions (Figures S4S11), DFT pore size analyses (Figure S12), 1H NMR spectra (Figure S13), WAXS raw data (Figure S14), and SEM images (Figures S15-S21). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Author email addresses

[email protected] Corresponding Author

[email protected]

ACKNOWLEDGMENT The project was supported by the Swedish Research Council (grant number 2016-03568).

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Synopsis Biomass conversion processes often form solid by-products known as humins. This work shows that humins can be highly porous and useful towards applications.

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Microporous humins prepared from sugars and bio-based polymers in

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