Zirconium-Incorporated Mesoporous Silicates Show Remarkable

Research Article pubs.acs.org/journal/ascecg

Zirconium-Incorporated Mesoporous Silicates Show Remarkable Lignin Depolymerization Activity Kakasaheb Y. Nandiwale,†,‡ Andrew M. Danby,† Anand Ramanathan,† Raghunath V. Chaudhari,†,‡ and Bala Subramaniam*,†,‡ †

Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W. 15th Street, Lawrence, Kansas 66045, United States

‡

S Supporting Information *

ABSTRACT: The depolymerization of dealkaline lignin in CH3OH + H2O mixtures was investigated over various solid acid catalysts, including commercial zeolites (H-FER, H-MOR, H-Beta, H-USY, and H-ZSM-5) with predominantly Brønsted acid sites and zirconium-incorporated mesoporous silicates (Zr-KIT-5 and Zr-KIT-6 with different Si/Zr ratios) with predominantly Lewis acid sites. Gel permeation chromatography (GPC) analyses of the product mixture, obtained from half hour batch reactions on the various catalysts at 250 °C and under a nitrogen pressure of 0.7 MPa, revealed a clear decrease in the molecular weight distribution, compared to the parent lignin. The yields of aromatic monomers (7.5 wt % of the initial lignin) and overall soluble aromatic species (∼77%) observed over Zr-KIT-5 is higher, compared to those observed with H-ZSM-5 (4.1% and 67%, respectively) under identical conditions. Furthermore, the yield of catalyst deposits is lower in Zr-KIT-5 (14%), compared to HZSM-5 (27%). The predominant Lewis acidity of Zr-KIT-5 appears to reduce aldehyde condensation reactions promoted by the Brønsted acid sites present in zeolites. However, a lack of hydrothermal stability of the mesoporous silicates is a drawback that must be overcome for practical viability. KEYWORDS: Lignin deconstruction, Acidity, Mesoporous silicates, Zirconium

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monomers at 150−300 °C. Ni/C and Pt/C catalysts in conjunction with hydrogen donor solvents, such as methanol and formic acid, have been shown to depolymerize lignin.15−17 Several lignin deconstruction strategies have also been reported recently, using catalytic hydrodeoxygenation.18 Oxidative lignin depolymerization has also been receiving increased interest, using either H2O2 or O2 as an oxidant over various catalysts, viz., methyltrioxorhenium,19 Pd/γ-Al2O3,20 ionic liquids (butylimidazolium hydrogen sulfate and triethylammonium hydrogen sulfate),21 MgO,22 perovskite-type oxide,23 and CuO/Fe2(SO4)3/NaOH.24 Homogeneous metal complexes and salts of Ru and Mn have been reported to depolymerize lignin model compounds and lignin.25−29 Rahimi et al.,30 developed metal-free oxidation strategies for lignin and lignin model compounds. Recently,

INTRODUCTION Significant amounts of lignin are generated annually by the paper/pulp industry.1 The emerging cellulosic ethanol industry is also expected to generate vast amounts of lignin. The deconstruction of lignin into aromatic platform chemicals is essential for facilitating the economic viability of cellulosic ethanol biorefineries. However, such lignin valorization remains a challenge, because of the recalcitrance caused by its heterogeneity, diversity of interaromatic ring linkages, and complex three-dimensional polymeric structure composed of C−C and C−O bonds.2−6 In recent years, several catalytic and thermochemical approaches for lignin depolymerization have been explored. Hydrogenolysis of lignin, involving simultaneous depolymerization and hydrogenation, has been reported over heterogeneous metal catalysts with externally added H2.7−9 This includes supported metal catalysts such as Pd/C,10 H-BEA-35/Raney Ni,11 Ni/C,12 Pt/Al2O3,13 and nano-SiC.14 In these experiments, lignin is reported to yield up to 50 wt % aromatic © 2017 American Chemical Society

Received: April 29, 2017 Revised: June 25, 2017 Published: June 28, 2017 7155

DOI: 10.1021/acssuschemeng.7b01344 ACS Sustainable Chem. Eng. 2017, 5, 7155−7164

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ACS Sustainable Chemistry & Engineering

samples were measured using a Panalytical Empyrean diffractometer using Cu K radiation at 45 kV and 40 mA. Nitrogen sorption isotherms were obtained at −196 °C on an Autosorb-iQ analyzer (Quantachrome, USA). The specific surface area (SBET) and pore diameter (dp,BJH) of the samples were estimated using the Brunauer− Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively. The amount of nitrogen physisorbed at P/P0 = 0.98 was used to estimate the total pore volume (Vp,BJH) of the catalyst samples. The total acidity of all catalysts was measured via temperatureprogrammed desorption of ammonia (NH 3 -TPD), using an AutoChem 2910 instrument (Micromeritics, USA) that was equipped with a thermal conductivity detection (TCD) device. Prior to the measurements, the samples were dehydrated at 250 °C in flowing He (30 sccm).48 Catalytic Lignin Depolymerization Studies. Catalyst testing was performed in a 300 mL batch reactor (Parr Instruments) with both unwashed and washed lignin. In a typical experiment, the reactor was loaded with the catalyst (0.1 g typically) and lignin (1.5 g typically) dissolved in a solvent mixture composed of methanol CH3OH (75 mL) and H2O (15 mL). The reactor was sealed, purged with 1 MPa N2 three times, and pressurized with 0.7 MPa of N2 at ambient temperature. The reactor was then heated to 250 °C with initial stirring at 100 rpm. Once the desired temperature was attained, usually within 25 min, the stirring speed was increased to 1000 rpm, signifying the start of reaction. Following the fixed-time batch run, the heating and stirring were simultaneously stopped. The reactor was allowed to cool in air for 5 min before immersing in an ice−water mixture for ∼10 min, in order to rapidly cool the reactor contents to room temperature. To test for possible aromatization of the methanol present in the solvent mixture, a blank reaction (without lignin) with CH3OH + H2O (75 + 15 mL) and H-ZSM-5(11.5) (0.1 g) catalyst was performed. Separation and Analysis of Products from Catalytic Lignin Depolymerization Reactions. Figure 1 displays the schematic of the

alkaline catalysts, such as NaOH, KOH, and CsOH have also been used in base-catalyzed depolymerization (BCD) of lignin into aromatic monomers.31−34 The main drawbacks of BCD are the relatively low selectivity toward aromatics, use of harsh reaction conditions, requirement of a neutralization step, and reactor corrosion.35 To overcome these drawbacks, solid base catalysts such as porous metal oxides containing hydrotalcites,36,37 Ni-supported layered double hydroxide hydrotalcite (Ni-HTC),38 nitrate-intercalated hydrotalcite,7 and Al and zeolitic catalysts (NaX, NaY, NaP) 39 have been investigated for lignin depolymerization. Solid acid catalysts such as commercial zeolites have also been reported for the depolymerization of dealkaline lignin.35 Recent studies on the depolymerization of hydrolyzed lignin utilized Al-SBA-15 and Ni-SBA-15 catalysts.40 In this work, we report the use of predominantly Lewis acidic, Zr-incorporated mesoporous silicates (Zr-KIT-5 and ZrKIT-6) for the depolymerization of a dealkaline lignin sample. This work is motivated by several factors. First, homogeneous Lewis acidic catalysts such as metal chlorides,41,42 metal acetates,41 and metal triflates41,43,44 have been shown to be effective in the selective cleavage of ether linkages in lignin. Furthermore, Brønsted acid sites are known to promote undesired condensation of the depolymerized products that may form during lignin deconstruction.45 Therefore, we hypothesized that the use of a predominantly Lewis-acidic solid catalyst should increase the yield of the monomeric products, compared to a predominantly Brønsted acidic catalyst such as H-ZSM-5. The Zr-KIT-5 and Zr-KIT-6 catalysts are easily synthesized using one-pot techniques and are effective for Lewis-acid catalyzed reactions such as anisole benzylation46 and isopropanol dehydration.47 Through fixed-time batch runs under identical operating conditions (250 °C, 0.7 MPa N2, substrate/catalyst ratios of 1−15), we compared the relative performances of various catalysts ranging from predominantly Brønsted acidic catalysts (commercial zeolites including HZSM-5) and predominantly Lewis acidic catalysts (Zr-KIT-5 and Zr-KIT-6), wherein the Lewis acidity is tuned easily by varying Zr loading.47,48 While the results provide clear evidence of the superiority of the Zr-based catalysts to depolymerize lignin, they also point to the lack of hydrothermal stability of these catalysts and identify the challenges/opportunities for designing stable catalysts.

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EXPERIMENTAL SECTION

Materials. Dealkaline lignin was procured from TCI Chemicals (Product No. L0045). When used as received, the dealkaline lignin is referred to as unwashed lignin. The dealkaline lignin was purified by washing with distilled water (100 mL distilled water/g of lignin), followed by filtration to remove water-soluble inorganics. The filtered lignin is then dried overnight under vacuum at 100 °C. Lignin obtained via this method is termed as washed lignin. Tetrahydrofuran (THF, 99.9%), methanol (99.9%), and dimethylformamide (DMF, 99.9%) were purchased from Fisher Scientific. Tetraethyl orthosilicate (TEOS, 98%), zirconium(IV) oxychloride octahydrate (99.5%), and pluronic P123 (EO20−PO70−EO20) were obtained from Aldrich. The commercial zeolitesH-FER, H-MOR, H-Beta, H-USY and H-ZSM-5 with different Si/Al ratioswere procured from Zeolyst International. The zeolite samples were calcined in air at 550 °C for 5 h prior to evaluation. Catalyst Synthesis and Characterization. Physical characterization results for the commercial zeolites are summarized in Table S1 in the Supporting Information. The Zr-KIT-546 and Zr-KIT-648 catalysts were prepared by previously reported procedures. Small-angle and wide-angle X-ray diffraction (XRD) patterns of the catalyst

Figure 1. Schematic representation of product fractions isolated from lignin depolymerization. various product fractions isolated from lignin depolymerization and the terminology used to describe them. The solid phase obtained following the reaction was separated from the liquid by filtration. The filter cake consists of the spent catalyst and a solid product residue. The filter cake was dried overnight in a vacuum oven at 100 °C. The weight of the solid product residue (WR) was estimated by subtracting the weight of catalyst charged (WC) from the weight of the dried filter cake (WSC). The solvent (CH3OH + H2O) soluble fraction (SF) was then treated in a Buchi Rotavapor R-215 to evaporate the solvent, producing a solid phase (P). After determining the mass of this solid 7156


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ACS Sustainable Chemistry & Engineering phase (WP), it was extracted with tetrahydrofuran (THF) to yield THF-soluble products (TSP, WTSP). The insoluble fraction was separated by filtration and the resulting filter cake was dried overnight in a vacuum oven at 100 °C to obtain the THF-insoluble fraction (TIF, WTIF). We employed THF as an extracting solvent in order to follow an identical product workup procedure reported previously with H-ZSM-5 catalyst.35 In addition to enabling us to reproduce the reported results with H-ZSM-5, the use of THF also allowed us to benchmark the performance of this catalyst with the current Zr-KIT-5 catalyst in a proper manner. In our ongoing work, solvents such as γvalerolactone, ethyl acetate, and similar sustainable solvents are being used for product fractionation. Note that the lignin used in the present study is insoluble in THF. Hence, we consider the TIF fraction to be mostly unreacted lignin. The TSP mass (WTSP) was calculated based on the difference between the mass of solid phase (WP) and the mass of TIF (WTIF) recovered. The lignin conversion, yields of the various product fractions, and the overall mass balance closure were calculated based on the following equations (eqs 1−7). The uncertainties in the lignin conversion (XL), product yields (YR, YTIF, YTSP, and YIM) and mass balance closure (MBC) were obtained by carrying out experiments in duplicate and found to have a standard mean deviation of approximately ±3%.

yield of solid product residue, YR (wt %) = =

conductivity detector (GC-TCD). However, no gaseous products (hydrocarbons, CO, or CO2) were detected implying total selectivity toward liquid-phase products. Thermogravimetric analysis and differential thermogravimetric analysis (TGA-DTA) of unwashed and washed lignin samples were performed with a thermogravimetric analyzer (TA Instruments, SDT Q600). XRD patterns of unwashed and washed lignin were recorded with a Panalytical Empyrean diffractometer, using Cu K radiation at 45 kV and 40 mA. The overall sulfur content and its chemical identity within the lignin structure were obtained via X-ray fluorescence (XRF, Panalytical Zetium) and X-ray diffraction (XRD) instruments, respectively. Nuclear magnetic resonance (NMR) spectra of the TSP fractions were recorded using a 500 MHz spectrometer (Bruker, Model Avance AVIII) that was equipped with a dual carbon/proton (CPDUL) cryoprobe. Approximately 0.1 g of lignin or depolymerized lignin product was dissolved in 0.7 mL dimethyl sulfoxide-d6. For the 13C NMR analysis, an inverse gated decoupling sequence was used with the following parameters: 15 s relaxation delay, 64 K data points, and 768 scans. Two-dimensional (2D) 1H−13C HSQC (heteronuclear single quantum correlation) NMR spectra were obtained using the hsqcedetgpsisp2.2 pulse program. The spectral widths were 16 ppm (8012 Hz) and 165 ppm (20 833 Hz) for the 1H and 13C dimensions, respectively. A relaxation delay of 2 s was used with 48 scans and 512 time increments recorded in the 13C dimension. The 1JCH used was 140 Hz. The central solvent peak (DMSO) was used as an internal chemical shift reference point (δC/δH 39.5/2.49). The feed mixture containing lignin and the TSP fraction was also analyzed by gel permeation chromatography (GPC) with an Agilent 1260 Infiniti GPC system fitted with a refractive index detector. Two columns were used in series at 40 °C: a 300 mm Polargel-M, followed by a 300 mm Polargel-L. The samples were eluted with DMF at a flow rate of 1.0 mL/min. Poly(methyl methacrylate) (PMMA) standards were used for calibration. The C/H ratios of lignin and the products were calculated using a PerkinElmer Series II 2400 CHN/O analyzer. Catalyst Stability Testing. Catalyst stability was evaluated by performing batch depolymerization runs (with and without lignin) and by characterizing the spent catalysts recovered from these runs. Typically, 1 g of catalyst with 100 mL solvent (H2O/CH3OH (1/5 v/ v) or H2O) was taken in the reactor and heated to the desired temperature. Once the desired temperature was reached, the stirring rate was ramped up to 1000 rpm and continued for 0.5 h. The reactor was rapidly cooled by immersion in an ice water bath and the catalyst was separated from the solvent by filtration. The filtered catalyst was dried in vacuum at 100 °C overnight, calcined at 550 °C for 5 h and then characterized by N2-physisorption, NH3-TPD, XRF, and smallangle X-ray scattering (SAXS) techniques to discern any changes in the structure and acidity during the batch runs. Elemental analysis of fresh and spent Zr-KIT-5(20) catalysts was performed on a Horiba Jobin Yvon JY 2000 ICP-OES instrument.46

WSC − WC × 100 WL WR × 100 WL (1)

yield of solvent (methanol + water) soluble fraction, YP (wt %) W = P × 100 WL (2) mass balance closure, MBC (%) =

WR + WP × 100 WL

yield of THF‐insoluble fraction, YTIF (%) = lignin conversion, XL (%) =

WTIF × 100 WL

WL − WTIF × 100 WL

yield of THF‐soluble products, YTSP (%) =

yield of identified monomers, YIM (%) =

WTSP × 100 WL

WIM × 100 WL

(3)

(4)

(5)

(6)

(7)

The TSP fraction was analyzed by GC-FID and GC/MS using an Agilent 7890A GC system coupled to a Model 5975C MS, HPINNOWAX column with He as a carrier gas (1 sccm), an inlet temperature of 250 °C, and an injection volume of 1 μL. The oven temperature was initially held at 40 °C for 5 min, then ramped at a rate of 10 °C/min to 220 °C and held at this temperature for an additional period of 20 min. Masses were scanned over a range of 20−500 Da. Products were identified by matching retention times and MS fragmentation patterns with external standards. Fifteen (15) aromatic monomers were confirmed in this manner (see Figure S1 in the Supporting Information). The quantity of identified monomers in the product mixture was calculated from GC-FID chromatograms following calibration. Response factors were calculated using available monomer standards. For monomers without available standards, an average response factor obtained from standards was used. The total quantity of all identified monomers is designated as WIM and yield of identified monomers (YIM) is calculated by eq 7. The TSP fraction may also contain dimers, trimers, and other oligomers but these were not identified. The gas phase from a batch run was collected in a sampling bag and analyzed by cryogenic gas chromatography−thermal

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RESULTS AND DISCUSSION Characterization of Lignin Impurities. The TGA profile of unwashed lignin revealed ∼15 wt % of unburnt residue (Figure 2a). A similar quantity (15 wt %) of inorganic residue (ash) was obtained after lignin calcination at 650 °C for 6 h. XRF analysis of unwashed lignin confirmed the presence of inorganics such as Na and S. The XRD pattern (Figure S2 in the Supporting Information) of unwashed lignin revealed the presence of Na2SO4.35 The absence of XRD peaks for either amorphous cellulose (2θ = 17.7°−18.5°) or for crystalline cellulose (2θ = 22.5°) confirms that the lignin is devoid of cellulose contamination.35 To minimize the inorganic impurities in the lignin, the dealkaline lignin was washed with distilled water (100 mL/g lignin), followed by filtration to remove water-soluble 7157


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inorganics. The washed lignin was then dried overnight under vacuum at 100 °C. Figure 2b compares the XRD patterns of unwashed lignin (as received) and of washed lignin. It can be clearly seen that the peaks corresponding to the inorganics (Na2SO4) were absent in the washed lignin. However, XRF analysis of the washed lignin indicated the presence of Na and S. TGA profiles (Figure 2a) showed the presence of 3 wt % residue in washed lignin and, following calcination in air at 1000 °C, an 80% reduction, compared to the unwashed lignin. It is possible that these inorganics are chemically bound to the lignin framework and consequently are not amenable to removal by simple washing. Although washed lignin contains ∼3 wt % inorganics, the XRD profile of the washed sample did not indicate any peaks. This may be due to the inorganics being present either in the amorphous form or below the instrument’s detection limit. Depolymerization Studies with Unwashed Lignin. The lignin depolymerization activities displayed by the commercial zeolites and the Zr-based mesoporous silicates were characterized based on various product fractions shown schematically in Figure 1. The blank run with solvent (CH3OH + H2O) alone (without dissolved lignin) confirmed that no aromatic monomers were formed after 0.5 h over H-ZSM-5(11.5) catalyst at 250 °C. Initial experiments employed a rather large catalyst/lignin ratio (1.5 g catalyst per 1.5 g of lignin) in order to benchmark reported literature data for the H-ZSM-5 catalyst.35 In all subsequent runs, a lower catalyst loading (0.1 g catalyst per 1.5 g of lignin) was used. Table 1 summarizes key performance metrics for unwashed lignin deconstruction over various catalysts. The mass balance closure was >90% in all the experiments (see Table S2 in the Supporting Information). Thermal depolymerization in the absence of catalyst resulted in a lignin conversion (XL) of 46%, with the yield of THF-soluble products (YTSP) being ∼9 wt % (Table 1, entry 1). In contrast, the conversion of lignin on

Figure 2. (a) TGA profiles and (b) X-ray diffraction patterns of unwashed and washed lignin.

Table 1. Catalyst Performance for the Depolymerization of Unwashed Lignina Yields of Product Fractions entry

catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

no catalyst H-FER(27.5) H-MOR(45) H-Beta(150) H-Beta(19) H-Beta(12.5) H-USY(40) H-USY(30) H-USY(6) H-USY(2.6) H-ZSM-5(140) H-ZSM-5(25) H-ZSM-5(11.5) Si-KIT-6 Zr-KIT-6(100) Zr-KIT-6(40) Zr-KIT-6(20) Zr-KIT-5(100) Zr-KIT-5(40) Zr-KIT-5(20)

total acidity (NH3 mmol/g)

XL (%)

YTIF (wt %)

YTSP (wt %)

YR (wt %)

0.41 0.47 0.70 0.88 1.03 0.16 0.32 0.81 0.90 0.15 0.73 0.97 0.01 0.19 0.31 0.50 0.16 0.32 0.64

46.0 55.7 60.0 53.0 54.9 57.7 52.0 54.1 62.2 67.0 57.0 70.0 78.5 50.2 59.0 60.5 64.5 59.0 66.0 73.0

54.0 44.3 40.0 47.0 45.1 42.3 48.0 45.9 37.8 33.0 43.0 30.0 21.5 49.8 41.0 39.5 35.5 41.0 34.0 27.0

9.0 15.1 20.6 14.1 17.5 19.4 15.2 17.3 22.3 26.9 16.7 26.9 30.3 11.4 17.2 20.7 24.6 23.4 31.1 34.0

29.9 31.5 33.6 31.7 32.0 32.8 29.0 31.0 33.7 35.8 35.0 37.0 41.0 33.1 33.8 34.0 35.8 29.3 30.7 32.0

Reaction conditions: unwashed lignin (1.5 g), catalyst (0.1 g), H2O/CH3OH (1/5 v/v), 250 °C, 0.5 h, 1000 rpm, 0.7 MPa of N2 at ambient temperature. Mean standard deviation = ±3%.

a

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Table 2. Comparative Performance of H-ZSM-5(11.5) and Zr-KIT-5(20) for Depolymerization of Washed Lignina Yields of Product Fractions entry 1 2 3 4 5

substrate

XLb (wt %)

liquid fraction (SF) from first depolymerization of washed lignin

87.0 91.0 87.0 97.8 98.7

catalyst Zr-KIT-5(20) H-ZSM-5(11.5) no catalyst Zr-KIT-5(20) H-ZSM-5(11.5)

washed lignin

c

YTIF (wt %) YTSP (wt %) YR (wt %) 13.0 9.0 13.0 2.2 1.3

66.0 59.0 66.0 76.8 66.7

14.0 27.5 14.0 14.0 27.5

YIM (wt %) 2.0 1.8 2.0 7.5 4.1

Reaction conditions: washed lignin (1.5 g), catalyst (0.1 g), H2O/CH3OH (1/5 v/v), 250 °C, 0.5 h, 1000 rpm, 0.7 MPa of N2 at ambient temperature. Standard deviation about mean = ±3%. bXL, YTIF, YTSP, and YIM values were calculated based on lignin charged in the first batch run. c Washing of dealkaline lignin was performed with distilled water (in the proportion 100 mL/g of lignin) followed by filtration to remove watersoluble inorganics and drying the lignin under vacuum at 100 °C overnight. a

yield of the solid residue (YR) observed with Zr-KIT-5(20) also decreased from 32% (Table 1, entry 20) to 14% (Table 2, entry 1). In contrast, the YR value observed with H-ZSM-5(11.5) was significantly greater (28%; see Table 2, entry 2), suggesting that the Brønsted acidic sites possibly promote the recombination of the initial depolymerization products to form heavier products. The filtered product mixtures from the first batch runs (SF) with washed lignin (Table 2, entries 1 and 2) were subjected to subsequent batch runs under identical operating conditions (250 °C, 0.5 h, 0.7 MPa N2), one without catalyst and the other by adding 0.1 g of the respective fresh catalyst. While the run without catalyst (Table 2, entry 3) showed little, if any, change in the product distribution, the run with the catalyst resulted in almost complete lignin conversion with no increase in the solid product residue (YR) over either Zr-KIT-5 or H-ZSM-5 catalysts (Table 2, entries 4 and 5). This implies that reducing the inorganic impurities in the lignin eliminates char formation on the catalyst. However, the yield of the THF-soluble products (YTSP = 77%) obtained with Zr-KIT-5(20) was higher than the corresponding value (67%) obtained with H-ZSM-5(11.5). When considering only the identified monomers, a higher yield of such monomers was obtained over Zr-KIT-5(20) (YIM = 7.5%; Table 2, entry 4) compared with H-ZSM-5(20) (YIM = 4.1%; Table 2, entry 5). Among the identified monomers, the yield of vanillin (2.3%) obtained with Zr-KIT-5(20) is almost twice that (1.1%) obtained with H-ZSM-5(11.5). It has been recently reported that the aldehydes formed during the depolymerization of model lignin compounds condense on acidic sites.43,44 Repolymerization of monomers to dimers and oligomers is known to be favored with strong acid catalysts.9,13,40 Evidence from previously reported ultraviolet-visible light (UV-vis) spectroscopy studies of the Zr-KIT-5 catalyst46 shows that most of the Zr is incorporated as Zr4+ ion in a tetrahedral configuration. Indeed, Zr atoms that isomorphically substitute Si in a silicate would lead to Lewis acidity. Fourier transform infrared (FTIR) spectra of adsorbed pyridine46 confirm that ZrKIT-5 contains predominantly Lewis (L) acid sites and only a minor amount of Brønsted (B) acid sites (L/B ≈ 9). Therefore, it seems plausible that aldehyde condensation might be significantly higher on Brønsted acidic H-ZSM-5 compared to the predominantly Lewis acidic Zr-KIT-5. This might also explain the higher yield of catalyst deposits (YR) on the HZSM-5 catalyst. Characterization of Lignin Depolymerization Products. Approximately 25 monomers were identified by GC-MS in the depolymerized, THF-soluble product mixture, of which 15 monomers were quantitatively analyzed using external standards. Most of the identified products are consistent with

zeolite catalysts (Table 1, entries 2−13) ranged from 52% to 78% with significant YTSP yields (up to 30 wt %), which includes several phenolic monomeric species (Figure S1). Thus, a majority of the identified aromatic monomers are oxygenated species. The yield of the identified monomers (YIM) ranged from 2.1% to 3.2% (Table S2). Within the same family of zeolites, increases in XL and YTSP were generally observed with an increase in the acidity of the zeolites. For example, in the case of H-USY zeolites (Table 1, entries 7−10), as the acidity increased from 0.16 NH3 mmol/g to 0.90 NH3 mmol/g, the XL increased from ∼52% to 67% and the YTSP value corresponding increased from 15% to 27%. Interestingly, with H-ZSM-5 catalysts (Table 1, entries 11− 13) significant increases in both XL (from 57% to 78%) and the corresponding YTSP values (from 17% to 29%) were observed with increasing acidity from 0.15 NH3 mmol/g to 0.97 NH3 mmol/g. Overall trends in YTSP values were found to rank in the following order: H-ZSM-5 (11.5) > H-FER (27.5) > HMOR (45) > H-Beta (12.5) > H-USY (2.6). All Zr-KIT-6 catalysts (Table 1, entries 15−17) and Zr-KIT5 catalysts (Table 1, entries 18−20) tested were found to produce higher YTSP values, compared to the siliceous KIT-6 (Si-KIT-6) catalyst (entry 14). Generally, XL and YTSP values increase with Zr loading (i.e., total acidity), more so in the case of Zr-KIT-5 catalysts. The maximum XL and the maximum YTSP attained with these catalysts are similar to those obtained with zeolites. As shown in Table 1, the yields of THF-soluble products (YTSP) and identifiable monomers (YIM) obtained with unwashed lignin are comparable for Zr-KIT-5(20) [YTSP = 34% (Table 1, entry 20), and YIM = 3.4% (entry 20 in Table S2] and H-ZSM-5(11.5) [YTSP = 30.3% (Table 1, entry 13) and YIM = 3.2% (entry 13 in Table S2)]. Interestingly, the overall yield of solid residue (YR) obtained over H-ZSM-5(11.5) [YR = 41%] (Table 1, entry 13) was significantly higher than that observed with Zr-KIT-5(20) [YR = 32%] (Table 1, entry 20). This is attributed to the higher lignin conversion obtained with HZSM-5(11.5) [XL = 78.5%] (Table 1, entry 13) compared to Zr-KIT-5(20) [XL = 73%] (Table 1, entry 20) and the propensity to form more condensation products on H-ZSM-5, as discussed in the following section. Depolymerization Studies with Washed Lignin. The relative performances of H-ZSM-5(11.5) and Zr-KIT-5(20) catalysts in the depolymerization of washed lignin are summarized in Table 2 and in Table S3 of the Supporting Information. Compared to the unwashed lignin, both catalysts showed better performance with increases in lignin conversion and a doubling of the yield of the THF-soluble fraction (YTSP = 66% for Zr-KIT-5 and 59% for H-ZSM-5). Furthermore, the 7159


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(Figure 3b) reveal a clear decrease in the average molar mass of the THF-soluble product fractions from both the H-ZSM5(11.5) and Zr-KIT-5(20) catalysts compared to the initial lignin feed. The THF-soluble products possess molecular weights in the 10−11 kDa range. However, the product mixtures from H-ZSM-5(11.5) experiments show a larger fraction of the heavier products, compared to those with the Zrbased catalysts. Similar results were observed from GPC spectra obtained with unwashed lignin over Zr-KIT-5(20) and H-ZSM5(11.5) catalysts (see Figure S3 in the Supporting Information). The C/H ratios of the THF-soluble products (8.5) and the THF-insoluble products (8.6) obtained with Zr-KIT-5(20) and H-ZSM-5(11.5) catalysts are similar (Table 3). The lower

those expected from lignin depolymerization and also reported in the literature.35 A sample GC chromatogram and a list of identified monomers (IM) are given in Figure S1 in the Supporting Information). The GPC data are calibrated relative to PMMA standards and therefore do not give absolute molecular weights. Figure 3a

Table 3. C/H Ratio of Depolymerized Products Obtained over Zr-KIT-5(20) and H-ZSM-5(11.5) C/H Ratioa depolymerized products

Zr-KIT-5(20)

H-ZSM-5(11.5)

THF-soluble products, WTSP THF-insoluble products, WTIF solid product residue, WR

8.5 8.6 10.5

8.5 8.6 11.3

a

C/H ratios: unwashed lignin = 9.4, benzene = 12, toluene = 10.5, xylene = 9.6, naphthalene = 15, and anthracene = 16.8.

average C/H values, when compared to aromatic monomers such as benzene (12), toluene (10.5), xylene (9.6), naphthalene (15), and anthracene (16.8), suggest that the product mixture contains molecules with substantial aliphatic components and a low abundance of polyaromatic compounds. Furthermore, the higher C/H ratio of the solid residue (Table 3) formed over HZSM-5(11.5), compared to Zr-KIT-5(20) catalyst (11.3 vs 10.5) also points to the formation of heavier products over HZSM-5. Figure 4a shows the 2D HSQC NMR spectrum of the unwashed lignin. The chemical shift observed in the 3−5.5 ppm (1H) and 50−90 ppm (13C) regions are attributed to the β−β, β-O-4, β-5, and α-O-4 inter unit linkages in lignin.49 Peaks observed between 6−8 ppm (1H) and 100−140 ppm (13C) correspond to the aromatic region of the respective subunits. The chemical shifts in the region of 3.2−4.2 ppm (1H), 56−58 ppm (13C) are attributed to the different methoxy linkages of the syringyl and guaiacyl subunits. Figure 4b shows the 2D HSQC NMR spectrum of the THFsoluble product fraction obtained over Zr-KIT-5(20). Following reaction, the signals belonging to the aliphatic region corresponding to guaiacolics and alkylphenolics were found to increase significantly. Given that NMR signals of polymeric materials are subject to significant line broadening, this observed increase in signal intensity is probably due to the increased NMR susceptibility of the smaller molecules of the depolymerization products. New peaks corresponding to alkenes attached to aromatic side chains as well as the aromatic regions corresponding to guaiacolics and phenolics were also observed. Furthermore, new signals corresponding to aldehydes were also evident in the spectrum. These results confirm the cleavage of interunit linkages of lignin to produce aromatic monomers over the Zr-KIT-5(20) catalyst. Figure S4 in the Supporting Information shows the 1H NMR spectra of the unwashed lignin and the THF-soluble product fraction obtained with Zr-KIT-5(20) catalyst.

Figure 3. Gel permeation chromatography (GPC) spectra of (a) unwashed lignin and solvent soluble fraction (SF) obtained by depolymerization of unwashed lignin with no catalyst (Table 1, entry 1) and, (b) washed lignin and THF-soluble products obtained over HZSM-5(11.5) and Zr-KIT-5(20) catalysts.

shows GPC spectra of the depolymerized product mixture obtained from the noncatalytic run of unwashed lignin (Table 1, entry 1). Clearly, there is a 1−2 orders of magnitude reduction in the molecular weight distribution (MWD) even without the catalyst. The presence of the catalyst causes a further substantial reduction in the MWD (Figure 3b). Therefore, we hypothesize that lignin depolymerization first occurs thermally and on the outer catalyst surface. The products of these depolymerization steps are small enough to access the sites within the catalyst pores, resulting in further depolymerization. Figure 3b presents GPC spectra of the washed lignin, the THF-soluble product fractions obtained from the first catalytic runs with washed lignin, and the THF-soluble fractions from the second catalytic runs (performed with fresh catalysts) with the filtered product from the first batch runs. The GPC spectra 7160


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of unwashed lignin at 200 °C showed significant decreases in surface area and total pore volume, from 874 m2/g and 0.77 nm to 265 m2/g and 0.32 nm, respectively. The total acidity also decreased from 0.64 to 0.11 NH3 mmol/g. At 250 °C, the decrease in surface area following neat runs (without lignin) were more severe, compared to 200 °C, indicating a lack of hydrothermal stability at the higher temperature. Furthermore, only 5%−10% of the initial acidity was retained (Table 4). The N2 adsorption−desorption isotherm of fresh Zr-KIT-5(20) exhibited a type IV adsorption isotherm (see Figure S5 in the Supporting Information). It showed a sharp capillary condensation occurring between P/P0 = 0.60 and P/P0 = 0.70 and a broad H2-type hysteresis loop with desorption occurring at P/P0 ≈ 0.47.46 Following treatment in (CH3OH + H2O) solvent at 200 °C, the Zr-KIT-5(20) catalyst exhibited minor variations in the textural properties while maintaining the large uniform cagelike pores (Table 4, entries 2−4). Decreases in the total acidity and changes in the pore size distribution were observed with an increase in catalyst treatment temperature from 100 °C to 200 °C (see Table 4 (entries 2−4) and Figure S5a). However, the catalyst treated at 250 °C in both H2O and in (CH3OH + H2O) solvents exhibited a significantly different slope of the capillary condensation step, signifying partial structural degradation (Figure S4). The surface area and total acidity of the spent catalysts were also reduced (Table 4), indicating a lack of hydrothermal stability at 250 °C. Similar to the Zr-KIT-5(20) catalyst, the spent H-ZSM5(11.5) catalyst following treatment in CH3OH + H2O solvent between 150 °C and 250 °C also exhibited decreases in surface area and acidity (see Table 5, entries 2−4). With just water as solvent at 250 °C, H-ZSM-5(11.5) showed a major decrease in surface area and acidity (Table 5, entry 5). However, the decrease in surface area is

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