Tailoring ZSM-5 Zeolites for the Fast Pyrolysis of Biomassto Aromatic HydrocarbonsThomas C. Hoff,[a] David W. Gardner,[a] Rajeeva Thilakaratne,[b] Kaige Wang,[b]

Thomas W. Hansen,[c] Robert C. Brown,[b] and Jean-Philippe Tessonnier*[a]

Introduction

The fast pyrolysis of lignocellulosic biomass representsa simple, cheap, and efficient approach to produce bio-basedfuels and chemicals from renewable feedstocks.[1] In this pro-cess, solid biomass is heated to a high temperature (500–700 8C) to be converted thermochemically to light gases (CO,CO2), solid char, and organic vapors, which can be further con-densed to obtain the desired liquid bio-oil.[2] The ratio betweenthe gas, liquid, and solid fractions is particularly sensitive tothe heating rate. Fast heating rates on the order of 1000 8C s�1

are required to achieve bio-oil yields of 60–70 %.[3] The mainbyproducts are CO, CO2, and H2O, which result from decarbon-ylation, decarboxylation, and dehydration. These deoxygena-tion reactions are desired as they increase the energy densityof the liquid fraction, thus its potential as a biofuel.[4] Fast py-rolysis is also attractive because this versatile technology canaccommodate a wide range of feedstocks, which includeswood, switchgrass, and agricultural waste (e.g. , corn stover).However, bio-oil is a complex mixture of more than 300 oxy-

genated compounds, such as anhydrosugars, organic acids, al-dehydes, ketones, furanics, and phenolics,[5] and its highoxygen content and chemical complexity makes it unsuitablefor direct use as a biofuel. Additional processes that involveone or several heterogeneous catalysts are required to de-crease the oxygen concentration from ~45 % to less than 7 %and achieve stable blends with petroleum that allow refining.[6]

Various catalytic deoxygenation processes have been investi-gated and reviewed recently.[7] Integrated approaches in whichthe catalyst is mixed directly with the biomass are appealingas pyrolysis and deoxygenation occur simultaneously in thesame reactor. Notably, catalytic fast pyrolysis (CFP) using ZSM-5zeolite as a catalyst produces in a single step benzene, tolu-ene, xylene, and naphthalene, which can be used as buildingblocks by the petrochemical industry or further converted togasoline-range hydrocarbons using hydrogenation processesalready employed in refining.[8]

The isomorphous substitution of silicon with aluminumatoms in the well-defined crystal structure of zeolites gener-ates strong Brønsted acid sites (BAS), which can catalyzea broad range of cracking, isomerization, and alkylation reac-tions. The performance of a zeolite for a given reaction de-pends on its acid site density, pore size, and crystallographicstructure (pore network dimensionality, presence of largecages).[9] ZSM-5 is particularly desirable for reactions that in-volve small aromatics as its narrow pore size matches the dy-namic diameter of benzene. Consequently, only moleculeswith a similar size and shape can diffuse in or out of the crys-tal, which makes it an excellent catalyst for the production ofbenzene, toluene, para-xylene, and naphthalene.[8d]

The ZSM-5-catalyzed fast pyrolysis of cellulose to aromaticshas been investigated extensively.[2, 8d, 9b, 10] Despite many ef-forts, commercial ZSM-5 samples from Zeolyst Internationaloffer the highest reported yields of aromatic hydrocarbons to

The production of aromatic hydrocarbons from cellulose byzeolite-catalyzed fast pyrolysis involves a complex reaction net-work sensitive to the zeolite structure, crystallinity, elementalcomposition, porosity, and acidity. The interplay of these pa-rameters under the reaction conditions represents a majorroadblock that has hampered significant improvement in cata-lyst design for over a decade. Here, we studied commercialand laboratory-synthesized ZSM-5 zeolites and combined data

from 10 complementary characterization techniques in an at-tempt to identify parameters common to high-performancecatalysts. Crystallinity and framework aluminum site accessibili-ty were found to be critical to achieve high aromatic yields.These findings enabled us to synthesize a ZSM-5 catalyst withenhanced activity, which offers the highest aromatic hydrocar-bon yield reported to date.

[a] T. C. Hoff, D. W. Gardner, Prof. J.-P. TessonnierDepartment of Chemical and Biological EngineeringIowa State University617 Bissell Road, 2138 Biorenewables Research LaboratoryAmes, IA 50011 (USA)E-mail : tesso@iastate.edu

[b] R. Thilakaratne, Dr. K. Wang, Prof. R. C. BrownBioeconomy InstituteIowa State University617 Bissell Road, 1140E Biorenewables Research LaboratoryAmes, IA 50011 (USA)

[c] Dr. T. W. HansenCenter for Electron NanoscopyTechnical University of DenmarkKgs. Lyngby, 2800 (Denmark)

Supporting Information and the ORCID identification number(s) for theauthor(s) of this article can be found under http://dx.doi.org/10.1002/cssc.201600186.

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date and, therefore, these catalysts were employed in moststudies published recently.[11] The reason for this excellent per-formance has not been identified yet, and the lack of struc-ture–activity correlations currently constitutes a major barrierfor the rational design of ZSM-5 catalysts for CFP.

Several studies were aimed to further improve the aromaticsyield by enhancing diffusion and by passivating the outer sur-face of ZSM-5, two approaches used commonly in petroche-mistry.[8d, 10a, c, e, 12] Zheng et al. hypothesized that the slow diffu-sion of reactants and products in the ZSM-5 micropores is themain limiting factor to achieve a high performance.[10e] There-fore, they proposed to shorten the diffusion path by decreas-ing the size of the ZSM-5 crystals. The authors compared 2 mm,200 nm, and 50 nm crystals. Unfortunately, the results wereambiguous as the 200 nm crystals showed the highest aromat-ic yield but the 50 nm ZSM-5 gave the highest yield of desiredbenzene, toluene, and xylene (BTX) products. Additionally, re-action residence times were 50 s, which thus diminished anybenefits from improved diffusion. Modest improvements in theoverall aromatic yield were observed after the introduction ofmesopores in the zeolite crystals by desilication.[12b] Finally, pas-sivation of the zeolite outer surface by silylation and dealumi-nation was attempted to decrease the undesired conversion ofpyrolysis vapors to coke on extra-framework aluminumsites.[10a] However, these postsynthetic modifications did notimpact the catalytic performance significantly either.

Here, we synthesized and fully characterized a series of ZSM-5 catalysts with different elemental compositions, crystal sizes,porosities, and acidities in an effort to identify structure–property–activity relationships. Through the investigation ofthese samples and comparison with commercial ZSM-5 fromZeolyst and Clariant, we show that crystallinity and extra-framework aluminum, parameters neglected in previous stud-ies, play a key role in catalyst performance. These findingsprompted us to investigate alternative synthesis methods. Aremarkable ZSM-5 catalyst that offered the highest aromatichydrocarbon yield to date was obtained.

Results and Discussion

ZSM-5 with a controlled particle size and mesoporosity wassynthesized using a procedure developed by Petushkovet al.[13] This method produces ZSM-5 nanocrystals (primaryparticles) of 5.5–40 nm that self-organize into mesoporous ag-gregates (secondary particles) of approximately 200 nm. Themesopore surface area and volume can be tailored for thesesamples by varying the hydrothermal treatment temperaturebetween 130 and 190 8C if the gel composition is kept con-stant.[13] The obtained zeolites were characterized fully to es-tablish clear relationships between the catalytic activity andcatalyst properties, specifically crystallinity, elemental composi-tion, porosity, and acidity.

Catalyst characterization

SEM images (Figure 1) revealed that the ZSM-5 samples syn-thesized at 130–190 8C were homogeneous and composed of

nanocrystals organized in 200–600 nm aggregates, in goodagreement with the study of Petushkov et al.[13] The elementalcomposition of each sample was determined by X-ray energy-dispersive spectroscopy (EDS) using an accelerating voltage of15 kV. These conditions afforded a spatial resolution (analysisdepth) of approximately 2 mm sufficient to obtain bulk chemi-cal compositions for nanocrystalline samples. Measurementson commercial ZSM-5 of known chemical compositions con-firmed that the silica-to-alumina ratio (SAR) values calculatedfrom EDS analysis were accurate. The SAR values obtained forthe laboratory-synthesized nanocrystalline ZSM-5 samplesranged between 49 and 53 (Table 1). The only deviation wasobserved for the zeolite prepared at the lowest temperature(130 8C). A low temperature seemed to be detrimental to Al in-

Figure 1. SEM images of a–b) commercial Zeolyst ZSM-5 CBV2314 and hier-archical ZSM-5 samples synthesized at c–d) 130, e–f) 150, g–h) 170, and i–j) 190 8C for 24 h.

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corporation in the zeolite framework and resulted in a SAR of99.

Powder XRD patterns were acquired to study the crystalstructure of the samples and the presence of amorphous ma-terial (Figure 2). An internal standard was mixed with eachsample and used as a reference to calculate the relative crystal-linity of the zeolitic material. Only diffraction peaks characteris-

tic of the MFI framework type and internal standard were ob-served. In the present work, the relative crystallinity was calcu-lated using the intensity of the characteristic reflections insteadof the diffraction peak areas. Although both methods arecommon, the peak intensity is more sensitive to small varia-tions in crystal structure. Temperature was found to have a ben-eficial effect on the crystallization process in good agreementwith the results of Petushkov et al.[13] The intensity of the re-flections at 2 q= 23.08, 23.88, and 24.368 increased by 27 % ongoing from a 130 to 190 8C synthesis temperature. A lowercrystallinity was accompanied by an increase of the amorphousphase in the sample, as indicated by a more pronouncedamorphous scattering halo. Small peak shifts of 2 q= + 0.18were also observed for the least crystalline samples, for exam-ple, ZSM5-24-130, representative of a small contraction of theframework (smaller d spacing).

The nanostructuring of the catalyst increased the total sur-face area from 372 to 398–481 m2 g�1 (Table 1). A greater sur-face-to-volume ratio for these small crystals and their arrange-ment in aggregates resulted in a threefold enhancement ofthe mesoporosity compared to that of the commercial zeolite(Table 1). This increase is evident in the N2 physisorption iso-therms at high P/P0 and in the pore size distributions (PSD;Figure 3). Although the commercial ZSM-5 displayed a type IVisotherm with a narrow H4 hysteresis typical of microporousmaterials organized in disordered mesoporous aggregates, allsynthesized samples showed a more pronounced hysteresisloop characteristic of hierarchical materials.[15] The PSD (calcu-lated from the adsorption branch of the isotherm using theBarrett–Joyner–Halenda (BJH) model) revealed a broad distri-bution of mesopores from 5 to 50 nm for samples synthesizedat 130–150 8C, whereas higher synthesis temperatures favoredthe formation of more compact aggregates. Pores upwards of50 nm are significantly larger than those observed in MCM-41or SBA-15 and, therefore, diffusion is expected to be improvedsignificantly for the samples synthesized at 130 and 150 8C.

Changes in acidity were probed by ammonia temperature-programmed desorption (NH3-TPD; Figure 4) and FTIR spec-troscopy of pyridinated samples (pyridine-FTIR; Figure 5), twocomplementary techniques commonly used for zeolite charac-

Table 1. Synthesis conditions and characterization data for commercial and laboratory-synthesized ZSM-5 catalysts.

Catalyst[a] Conditions SAR[b] Surface area[c] [m2 g�1] Volume[d] [cm3 g�1] RC[e] 27Al FWHM[f] NH3-TPD BAS peak[g]

t [h] T [8C] Stotal Smicro Smeso Vtotal Vmicro [%] [ppm] center [8C] area [a.u.]

CBV2314 – – 23 372 274 98 0.202 0.127 100.0 5.9 408 86ZSM5-24-130 24 130 98.6 481 230 251 0.348 0.105 81.0 5.6 366 30ZSM5-24-150 24 150 49.2 438 254 184 0.364 0.117 86.7 5.8 387 39ZSM5-24-170 24 170 52.9 398 248 150 0.273 0.114 100.9 5.4 409 66ZSM5-24-190 24 190 52.3 421 243 178 0.291 0.111 102.5 5.3 413 68ZSM5-OPT 40 180 34.4 318 244 74 0.159 0.113 100.7 4.9 432 147

[a] CBV2314: commercial ZSM-5; ZSM5-24-x : nanocrystalline ZSM-5 synthesized with various hydrothermal treatment temperatures (x = 130–190 8C) usingthe method of Petushkov et al. ;[13] ZSM5-OPT: microcrystalline ZSM-5 synthesized using a recipe adapted from Kleinwort.[14] [b] Silica-to-alumina ratio calcu-lated from EDS analysis. [c] Specific surface areas determined from N2 physisorption using the BET (total) and t-plot (micropores) methods. The mesopo-rous surface area was calculated by difference. [d] Total and microporous volumes determined by N2 physisorption using the single-point adsorption porevolume (total) and t-plot (micropores) methods. [e] Relative crystallinity calculated based on the intensity of the main diffraction peaks. Results were nor-malized to the commercial CBV2314. [f] Full width at half maximum (FWHM) of the 27Al SSNMR spectroscopy peak that corresponds to framework Al.[g] Peak center and area for the contribution that corresponds to strong BAS in the NH3-TPD curves.

Figure 2. (top) Powder XRD patterns obtained for commercial (CBV2314)and nanocrystalline samples synthesized at various temperatures.(bottom) The addition of an internal standard allowed us to scale the pat-terns and compare characteristic MFI peaks.

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terization.[16] The NH3-TPD curves obtained for similar zeolitesmeasured under the same conditions provides valuable infor-mation on changes in the total (Lewis and Brønsted) acid sitedensity within a sample series.[16] A net increase in acidity withsynthesis temperature, independent of elemental compositionis illustrated in Figure 4. These results suggest a better alumi-num insertion in the zeolites. Although it is difficult to distin-guish Lewis from Brønsted acid sites by NH3-TPD, Bates et al.

demonstrated a direct correlation between the contribution at366–413 8C and N-propylamine decomposition.[17] Therefore,this TPD peak can be assigned unambiguously to strong BASassociated with framework Al atoms. An integration of thiscontribution (Table 1) supports an increase in BAS with synthe-sis temperature, in good agreement with the improved Al in-sertion in tetrahedral framework sites at the expense of amor-phous Al species. This interpretation is also consistent withpyridine-FTIR spectroscopy and 27Al solid-state nuclear magnet-ic resonance (SSNMR) spectroscopy (vide infra). In addition, theBAS peak center shifted from 366 to 413 8C with the increasingsynthesis temperature, which indicates the presence of stron-ger BAS in the more crystalline samples.

Changes in acidity were further investigated by pyridine-FTIR spectroscopy as this probe molecule generates distinct IR-active vibrations if chemisorbed on Lewis or Brønsted acidsites (Figure 5). Interestingly, the integration of the peak at n=

1455 cm�1 revealed similar concentrations of Lewis acid sites,independent of the synthesis parameters. The concentration ofBAS (n�1550 cm�1) increased with the synthesis temperaturefollowing the trend 130<150<170�190 8C. This trend is con-sistent with the changes observed in the 27Al SSNMR spectra(Figure 6 and Figures S1 and S2). The obtained SSNMR spectradisplayed two main peaks centered at d= 55 and 0 ppm,which corresponds to tetrahedrally coordinated framework alu-minum atoms (AlTd) and octahedral extra-framework Al species(AlOh), respectively. The isomorphous substitution of frameworkSi with Al in the tetrahedral coordination creates negativeframework charges that are balanced by protons to give zeo-lites their characteristic strong Brønsted acidity.[18] The linearcorrelation between BAS (H+) and tetrahedrally coordinated Alatoms allows the quantification of strong BAS in protonic zeo-lites by SSNMR spectroscopy.[17] In contrast to chemisorptiontechniques, SSNMR spectroscopy probes the total number ofstrong BAS (associated with framework Al), regardless of theiraccessibility. Therefore, the fact that SSNMR spectroscopy (AlTd

peak), NH3-TPD, and pyridine-FTIR spectroscopy share thesame 130<150<170�190 8C trend of the synthesis tempera-ture indicates that all the BAS are accessible and titrated in theequilibrated samples if NH3 and pyridine are given sufficient

Figure 3. (top) N2 isotherms and (bottom) PSD of commercial (CBV2314) andsynthesized hierarchical ZSM-5 samples.

Figure 4. NH3-TPD curves obtained for commercial (CBV2314) and the syn-thesized samples.

Figure 5. FTIR spectra of pyridinated samples. The peaks at n= 1550 and1455 cm�1 are characteristic of Brønsted and Lewis acid sites, respectively.

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time to diffuse inside the pore network. At this stage, it is alsoimportant to remember that the samples synthesized at 130and 150 8C present the highest mesopore surface area andvolume (Table 1). Yet, these samples are the least acidic. Theseresults indicate that our series of nanocrystalline ZSM-5 sam-ples is fundamentally different from the zeolites studied by Pu-�rtolas et al.[19] These authors identified a clear porosity–aciditycorrelation for mesoporous zeolites prepared by desilication.Hence, porosity, acid site density, and catalytic activity followedthe same trend. In contrast, all the techniques used in thepresent study indicate that the number of strong BAS increas-es with the synthesis temperature, whereas the amount ofamorphous material in the samples decreases, as indicated byXRD. Therefore, higher synthesis temperatures (170–190 8C) en-hance Al insertion in the zeolitic framework at the expense ofAl atoms involved in amorphous, NMR-invisible, extra-frame-work material, with an optimum at 170 8C for the gel composi-tion selected for this work.

The complex relationship between the characterized proper-ties reveals why clear structure–activity correlations have notyet been identified for zeolite-catalyzed fast pyrolysis. Thecombination of techniques used in this work is expected toprovide unique insights into these correlations.

Catalytic performance

The synthesized samples were tested for the CFP of celluloseto aromatic hydrocarbons. Yields and selectivities to the mostimportant products are reported in Figure 7. Notably, the CFPreaction is performed typically at a high temperature between600 and 800 8C. The optimal temperature (the temperaturethat affords the highest yields) varies, which depends on theconfiguration of the pyrolyzer used for the tests. Although py-roprobes are operated typically at 600–650 8C, micropyrolyzersperform better at 650–700 8C.[2b, 9b] Here, we chose to performthe reaction at 700 8C by using a micropyrolyzer on the basisof previous optimizations of our setup.[2b] Cellulose and catalystwere brought to the target temperature within 500 ms, andthe overall reaction proceeded within a few seconds. Only aro-

matic hydrocarbons were detected under these conditions, ofwhich benzene, toluene, xylene, and naphthalene accountedfor more than 70 % of the detected products.

Significant differences in activity were observed for thenanocrystalline zeolites with yields to aromatic hydrocarbonsin the range of 15–30 %. This broad difference in catalytic per-formance cannot be attributed to variations in the elementalcomposition as commercial ZSM-5 samples from Zeolyst andClariant with SAR values of 23 to 55 achieved similar yieldsunder our reaction conditions (27.5�1.0 %; Table S1). Interest-ingly, the laboratory-synthesized zeolites with the highest mes-oporosity (ZSM5-24-130 and ZSM5-24-150) performed very dif-ferently and gave yields of 15 and 27 %. These results are im-portant: although mesoporosity and a small crystal size mayenhance intracrystalline diffusion,[9a] other parameters playa more prominent role in the production of aromatic hydrocar-bons. This interpretation is consistent with previous work forwhich only minor improvements in BTX production were ach-ieved if mesopores were introduced in zeolite crystals.[12b]

A comparison of the reference CBV2314 with the hierarchicalZSM-5 synthesized at 150, 170, and 190 8C provided interestinginsights into the parameters that govern the catalytic activity.These four zeolites achieved similar yields (24–29 %) althoughthey exhibit a very different crystal size, aggregate size, porosi-ty, and acidity. Notably, ZSM5-24-170 achieved the same yieldas commercial ZSM-5 (CBV2314) although it had 25 % fewerBAS and a lower microporous volume, which were both report-ed to be critical to achieve a high aromatic hydrocarbon yieldfor the CFP of cellulose.[10a, e]

Key features and aromatic yields for the least active (ZSM-24-130) and the best catalysts (ZSM5-24-170 and CBV2314)were compared in a radar plot to identify key differences visu-ally and guide future rational catalyst design (Figure 8). Theoverlapping areas in the plot reveal that the best catalysts arehighly crystalline and present a strong acidity. These observa-tions were consistent for the commercial Zeolyst and Clariantzeolites (Table S2, Figure S3) as well as laboratory-synthesizedZSM-5. Surprisingly, mesoporosity (Smeso) and total surface area

Figure 6. 27Al SSNMR spectra of the commercial (CBV2314) and synthesizedZSM-5 samples. The peaks at d= 55 and 0 ppm are characteristic of Alatoms in framework and extra-framework sites, respectively. Figure 7. Aromatic yield and selectivity to the main aromatic hydrocarbons

obtained for the CFP of cellulose at 700 8C. The tests were performed byusing a micropyrolyzer equipped with online GC–MS analysis.

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(Stotal) do not seem to play a significant role in aromatic hydro-carbon production under our reaction conditions. New correla-tions also emerged between catalytic activity and AlTd NMRpeak intensity and shape. Correlations between AlTd peak in-tensity, acidity, and catalytic activity were identified and havealready been discussed in previous sections. However, thesecorrelations failed to explain why ZSM5-24-170 achieved thesame aromatic yield as commercial ZSM-5 with 50 % fewer acidsites. A more in-depth analysis of the SSNMR spectroscopy re-sults revealed interesting trends in the shoulder at d�50 ppm(Figure S1) and in the full width at half maximum of the AlTd

peak (Table 1). These observations could be consistent withthe presence of extra-framework amorphous silica-alumina inthe commercial zeolite as well as in the ZSM5-24-130 andZSM5-24-150 samples (also revealed by XRD).[20] Therefore, asa next step, we explored alternative gel compositions and hy-drothermal treatment conditions that favor the growth ofhighly crystalline ZSM-5 samples with strong acidity and en-hanced Al insertion in the zeolitic framework as these parame-ters seem critical to achieve high yields (vide infra).

Synthesis of ZSM-5 with enhanced CFP performance

The negligible amount of amorphous materials identifiable byXRD in samples synthesized at high temperature suggests thata significant increase in bulk crystallinity would be difficult toachieve. However, disordered surface species (e.g. , amorphousextra-framework silica-alumina) have been proposed to blocka vast majority (>99 %) of pore openings in small (<50 nm)MFI crystals.[21] These blockages are particularly difficult tocharacterize by aberration-corrected high-resolution transmis-sion electron microscopy (AC-HRTEM), AFM, and even chemi-sorption.[21] Although most acid sites remain accessible toprobe molecules under equilibrium conditions, frequency-response investigations demonstrated that these inorganicspecies hamper the diffusion of bulky molecules under reac-tion conditions. Clearly, these effects are expected to also takeplace for larger crystals, in particular for very fast reactionssuch as CFP. We hypothesized that tuning the synthesis condi-

tions to decrease the formation of these disordered specieswould also improve Al insertion into the zeolitic frameworkand enhance the catalytic activity.

Highly ordered (defect-free) zeolites are of particular impor-tance for membrane applications in which intercrystal diffusionpaths and varying pore sizes are detrimental to membraneperformance.[22] Research in this field has established that het-erogeneous nucleation growth techniques and extended crys-tal growth times are advantageous for single-phase MFI syn-thesis.[22] Through these synthesis techniques, we can minimizedefect formation and generate highly ordered crystals at theexpense of mesoporosity.

The synthesis of a highly ordered ZSM-5 catalyst was adapt-ed from a recipe by Kleinwort.[14] The method utilizes a seedingstep and long crystallization time to ensure a highly homoge-neous and crystalline ZSM-5. SEM images of the obtainedsample revealed microcrystals organized in aggregates of 3–5 mm, that is, approximately one order of magnitude largerthan the nanocrystals studied in the first part of this work(Figure 9). Characterization by XRD (Figure S4) confirmed that

the crystallinity of the sample was similar to commercial ZSM-5(relative crystallinity (RC) = 100.7 %). However, AC-HRTEM andselected area electron diffraction (SAED) showed differences inthe structure and amorphous content for the two samples(Figure 10 and Figures S5 and S6). Small (20–50 nm) crystallinedomains with a significant number of grain boundaries andlow-contrast areas that could correspond to amorphous re-gions were imaged for the commercial Zeolyst CBV2314. Thecorresponding SAED pattern was consistent as it showed thecoexistence of highly crystalline (bright spots) and amorphous(diffuse spots) regions. In contrast, the optimized ZSM-5 crys-tals have a well-aligned network of micropores that extendsover hundreds of nanometers. The high crystallinity of thesample was further confirmed by SAED.

As expected, the mesoporous surface area and volume wereminimal (sample ZSM5-OPT in Table 1). N2 physisorptionshowed a near-type I isotherm characteristic of microporousmaterials and the PSD displayed only few pores with a widthgreater than 1 nm (Figures S7 and S8). Hence, although the rel-ative crystallinity determined by XRD is similar for both sam-ples, the optimized ZSM-5 exhibits long-range order with mi-cropores free of any amorphous material.

The existence of pore blockages in both samples was furtherstudied by N2 uptake kinetic studies (Figure 11). These time-

Figure 8. Radar plot that highlights key structural and chemical features ofcommercial ZSM-5 CBV2314 and hierarchical ZSM-5 synthesized at 130 and170 8C.

Figure 9. a) Low- and b) high-magnification SEM images of the optimizedZSM-5 sample.

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resolved N2 adsorption experiments provide significant insightsinto the diffusion of small molecules with dynamic diameterswell below the pore size of the zeolite. The uptake experi-ments start after evacuating the samples and reaching a basepressure of 10 mmHg. Thus, the uptake kinetic traces providedirect information on the accessibility (and blockage) of themicroporous network of the zeolite. Diffusion in the commer-cial ZSM-5 is slow and the adsorbed volume reached a plateauafter �50 s (Figure 11). In comparison, the uptake for the opti-mized ZSM-5 was approximately one order of magnitudefaster despite the larger crystal and aggregate sizes (Figure 9).These experiments, together with AC-HRTEM images and SAEDpatterns, support the presence of an amorphous phase insidethe pores of the commercial zeolite and that may impact itscatalytic activity.

The optimized ZSM-5-catalyzed fast pyrolysis of celluloseproduced 32 % yield of aromatic hydrocarbons, a 12 % increasecompared to commercial CBV2314 tested under the same con-ditions (Figure 7). To the best of our knowledge, this is the first

time that the performance of Zeolyst ZSM-5 has been sur-passed. Notably, this excellent performance was obtained withmicroporous micron-sized crystals. Therefore, this experimentis consistent with the conclusions drawn from the first part ofthis work and confirms that although nanostructuring the zeo-lite crystals or inserting mesopores and mesovoids may helpinter- and intracrystalline diffusion, other parameters have a sig-nificantly more pronounced impact on the CFP activity and theformation of the desired aromatic hydrocarbons.

Further analysis of the microcrystalline ZSM-5 by NH3-TPD,pyridine-FTIR spectroscopy, and 27Al SSNMR spectroscopy (Fig-ures 12–14) confirmed that the synthesis conditions we usedto minimize defects also enabled a better insertion of Al in thezeolitic framework and, as a result, the formation of more ho-mogeneous and stronger BAS. Although the sample presentsa SAR of 34.4, thus a 50 % lower Al content than that of thecommercial ZSM-5, NH3-TPD revealed a significant increase instrong acid sites (Figure 12). Conversely, pyridine-FTIR spectros-

Figure 10. Aberration-corrected HRTEM images of a) commercial CBV2314and c) the optimized ZSM-5 sample. The corresponding SAED patterns (re-spectively, b and d) reveal a lower amount of amorphous species in the opti-mized zeolite.

Figure 11. N2 uptake curves for commercial and optimized ZSM-5.

Figure 12. NH3-TPD curve obtained for optimized ZSM-5. The curves thatcorrespond to commercial ZSM-5 and to the hierarchical ZSM5-24-170sample are also shown for comparison. The optimized sample exhibitsa more pronounced high-temperature desorption peak despite a lower Alcontent compared to commercial CBV2314. The peak is also shifted toa higher temperature, which is an indication of a stronger acidity.

Figure 13. FTIR spectra of pyridinated samples. Compared to the commercialzeolite used here as a reference, the optimized ZSM-5 exhibits only slightlylower Brønsted acidity despite a 50 % lower Al content by EDS analysis.

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copy displayed a similar increase, although less pronounced(Figure 13). This apparent discrepancy is most likely becauseCBV2314 exhibits weak acid sites that are captured by pyri-dine-FTIR spectroscopy but not by NH3-TPD because of theeasy desorption of ammonia. Extra-framework AlOH groupsare strong enough to retain pyridine at the desorption temper-ature used for pyridine-FTIR spectroscopy. However, the NH3

desorption activation energy for AlOH sites is lower than thatof BAS.[23] Therefore, AlOH would not appear in the strongestacid site region of TPD (above 350 8C). Further FTIR spectrosco-py studies using collidine and 2,6-di-tert-butylpyridine (DTBPy),two probe molecules too large to diffuse inside the ZSM-5 mi-cropore network, were performed to locate these AlOH sitesand obtain additional information on acid site accessibility.[24]

The absence of any signal for this sample series (Figure S9)demonstrates that the extra-framework AlOH species are locat-ed inside the pore network, in good agreement with the N2

uptake experiments. These measurements also ruled out anysignificant contribution from external acid sites and any porosi-ty–acidity correlation for CFP. This interpretation is also sup-ported by 27Al SSNMR spectroscopy results (Figure 14): thepeak that corresponds to tetrahedral aluminum increases in in-tensity and becomes narrower, which indicates more Al inhighly symmetric framework sites than for the commercialsample. We used the radar plot shown in Figure 15 to high-light the critical parameters for high catalytic activity and pro-vide further insight into the key factors that need to be furtheroptimized. A comparison of the results for commercial and op-timized ZSM-5 reveals that the increase in aromatic hydrocar-bon yield can be assigned to a higher Al ratio in frameworksites and, reciprocally, less Al in extra-framework surface spe-cies that block pores and, potentially, catalyze undesired reac-tions.

Conclusions

The ZSM-5 zeolite catalyzes the fast pyrolysis of cellulose witha high selectivity to small aromatic hydrocarbons (benzene, tol-uene, xylene, naphthalene), which find applications as bio-based chemicals or as gasoline-range fuels after additional hy-drogenation. The unique size and shape selectivity of ZSM-5

towards these compounds is well established and understood.However, the importance of other structural parameters forthe efficient transformation of pyrolysis vapors into aromaticsremained to be elucidated. It was proposed previously thatstrong Brønsted acid sites located inside the pores of the zeo-lite catalyze a series of deoxygenation, cracking, alkylation, andaromatization reactions. This hypothesis was based primarilyon analogies with the reactions of methanol to olefins andmethanol to hydrocarbons. Here, we have demonstrated thatamorphous silica-alumina surface species, even present insmall concentrations, impact the diffusion of bulky reactants,lower the amount of Al in framework sites, and consequently,alter the Brønsted acid site density and strength. These obser-vations were shown to hold without exception regardless ofthe catalyst manufacturer or synthesis method. On the basis ofthis finding, we designed a highly crystalline zeolite with mini-mal crystalline defects and amorphous material through theadaptation of techniques developed for zeolite membrane syn-thesis. This approach allowed us to further study the role ofzeolite crystallinity as well as the nature of its acid sites. Theyield to the desired products increased by 12 % and for thefirst time surpassed the aromatic hydrocarbon yield obtainedfor commercial ZSM-5 tested under the same conditions. Thiswork sets the foundation for future mechanistic studies andfor the design of new zeolitic materials optimized for catalyticfast pyrolysis.

Experimental Section

Catalyst synthesis

Reference ZSM-5 samples in their ammonium form were purchasedfrom Zeolyst International and used here for comparison:CBV2314, CBV3024E, CBV5524G, and CBV8014 with SiO2/Al2O3 = 23,30, 50, and 80, respectively. The samples were calcined in air at550 8C for 10 h (ramp: 5 8C min�1) before characterization and cata-lytic testing. ZSM-5 nanocrystals with controlled particle size andmesoporosity were synthesized according to a procedure pub-lished previously.[13] Briefly, a clear gel with the following molarcomposition of 25:1:5:4:1000 TEOS/NaAlO2/TPAOH/TPABr/H2O

Figure 14. 27Al SSNMR spectrum of optimized ZSM-5. The spectra of com-mercial ZSM-5 and ZSM5-25-170 are displayed for comparison. Figure 15. Radar plot that highlights key structural and chemical features of

commercial ZSM-5 and the optimized zeolite obtained in this work througha combination of seeded growth and extended crystallization times.

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[TPAOH = tetra-n-propylammonium hydroxide (Alfa Aesar, 40 %),TPABr = tetra-n-propylammonium bromide (Sigma–Aldrich, 98 %),TEOS = tetraethyl orthosilicate (Aldrich, 98 %)] was prepared. Onethird of the water, TPAOH, and sodium aluminate (Strem Chemi-cals, 99.9 %) were mixed together and stirred at 500 rpm for 5 minto ensure the complete dissolution of the aluminate. The remain-ing water and TPABr were then added, and the mixture was stirredfor an additional 5 min at 500 rpm. Finally, TEOS was mixed intothe solution and stirred overnight at RT in a closed polypropyleneflask. The resulting clear gel was loaded into a Teflon-lined Paarstainless-steel autoclave (Parr 4744) and placed in the middle ofa preheated mechanical convection oven (ThermoScientific Hera-therm OMS100) for 24 h. The synthesis temperature was variedfrom 130 to 190 8C. Following synthesis, zeolite crystals were col-lected by centrifugation (5000 rpm, 30 min) and washed twice withdeionized water and once with ethanol. After the final washing,the slurry was dried at 70 8C overnight. The sample was then cal-cined at 550 8C for 10 h (ramp: 5 8C min�1) to decompose the TPAstructure-directing agent. Finally, the acid form of the ZSM-5 wasobtained after three successive ion exchanges with a 0.5 m NH4NO3

(Fisher Scientific, ACS) solution at 70 8C, drying at 70 8C overnight,and calcination at 550 8C for 10 h. The optimized zeolite was syn-thesized according to the following procedure adapted from Klein-wort.[14] Seeding gel was prepared by adding NaOH (0.69 g) andTPAOH (5.85 g, 20 wt %) to deionized water (35.51 g) and stirring at500 rpm for 5 min. Silicic acid (7.945 g) was added slowly understirring and the solution was further stirred for 1 h at 500 rpm. Theseeding gel was then aged at 100 8C for 16 h. The synthesis gelwas prepared by mixing deionized water (86.78 g), NaOH (0.88 g),and sodium aluminate (1.03 g). The solution was stirred at 500 rpmfor 5 min. Silicic acid (11.31 g) was added slowly under stirring, andthe mixture was stirred for 1 h at 500 rpm. The seeding gel (5 g)was added to the synthesis solution and stirred for 1 h at 500 rpm.The final synthesis gel was placed in stainless-steel Teflon-lined au-toclaves, and crystallization occurred at 180 8C for 40 h. Followingsynthesis, samples were separated by centrifugation (5000 rpm for15 min) and washed twice with deionized water and once withethanol. The zeolite was then dried at 105 8C for 24 h. Calcinationand ion exchange procedures followed according to those usedfor the nanocrystalline samples.

Catalyst characterization

Powder XRD patterns were collected by using a Siemens D 500 dif-fractometer using CuKa radiation, a diffracted-beam monochroma-tor (graphite), and a scintillation detector. Data were recorded inthe 2 q range of 5–508 using a step size of 0.058 and a dwell timeof 3 s per step. The instrument broadening of the diffractionsystem was determined using the NIST LaB6 standard. All datawere analyzed using Jade software version 9.5. Test specimenswere prepared by mixing the bulk sample with an internal stan-dard (high purity corundum, Alfa Aesar, verified using NIST 674bstandards zincite, rutile, and cerianite). The mixture consisted of0.150 g of sample and 0.100 g of corundum. All measurementswere made by using an analytical balance and recorded to thenearest 0.1 mg. Then the components were mixed in an agatemortar-and-pestle. After mixing, the material was removed fromthe mortar, quickly recombined, and then placed back into themortar-and-pestle for a second mixing cycle. This produced a ho-mogeneous powder that contained 40 % internal standard bymass. Specimens for XRD analysis were prepared by placing 0.20�0.03 g of powder into the cavity of a zero-background holder (MTICorporation zero diffraction plate, size 20 mm diameter by 1 mm

deep). The powder was compacted into the cavity with a glassslide. Relative crystallinity was calculated by summing the peakmaxima for each sample at the characteristic peaks 2 q�23.08,23.88, and 24.368. Intensities are reported relative to the commer-cial sample (CBV2314) that was taken as 100 %.

N2 adsorption/desorption isotherms and N2 uptake were measuredby using a Micromeritics ASAP 2020 system at 77 K. Zeolitepowder (50–60 mg) was degassed at 200 8C (heating ramp:5 8C min�1) for 12 h under vacuum. The specific surface area wascalculated using the BET method. The BJH model with Faas correc-tion was applied to the adsorption branch of the isotherm to cal-culate the PSD. The t-plot method was used to discriminate be-tween micro- and mesoporosity. The N2 rate of adsorption experi-ments were performed by dosing 5 cm3 g�1 of N2 to a sampleunder vacuum (10 mmHg).

SEM images were acquired by using a FEI Quanta 250 FEG operat-ed at 10 kV. The samples were coated with 2 nm of Ir for conduc-tivity. X-ray analysis was performed by using an Oxford InstrumentsAztec� energy-dispersive spectrometer (EDS) system equippedwith an X-Max 80 detector. EDS spectra were typically recorded at15 kV, which corresponds to a beam penetration depth of �2 mm.

For HRTEM and SAED, the samples were dry-dispersed on a holeycarbon grid. Images and diffraction patterns were acquired byusing an FEI Titan 80–300 equipped with an aberration correctoron the objective lens. The microscope was operated at an accelera-tion voltage of 300 kV. To minimize the effect of the electronbeam, a low current density was used.

NH3-TPD was performed by using a Micromeritics Autochem II2920. Zeolite powder (50 mg) was pretreated at 550 8C (heatingramp: 10 8C min�1) in 10 mL min�1 He for 1 h to desorb any mois-ture from the surface. The sample was then cooled to 50 8C andammonia was adsorbed for 30 min (20 mL min�1 of 10 vol % NH3 inHe). The sample was then purged at 100 8C under flowing He for90 min. NH3 desorption was recorded by heating the zeolite from100 to 700 8C using a 10 8C min�1 ramp. Curves were normalizedusing the sample mass. Peak areas were determined using a Gaussanalysis in OriginPro 9.1 software.

FTIR spectroscopy was performed by using a Bruker Vertex 80spectrometer with a Harrick Praying Mantis diffuse reflection(DRIFTS) attachment. Samples were first pyridinated or adsorbedwith DTBPy for 48 h. Desorption occurred at 150 8C over 4 h forpyridine and 1 h for DTBPy to remove any physisorbed species. A2 % pyridinated zeolite/KBr mixture was made, mixed and groundby mortar-and-pestle, and sieved through a 45 mm sieve. DTBPysamples were ground by mortar-and-pestle and sieved througha 45 mm sieve. The samples were then analyzed using OPUS 7.0software. Absorbance from 4000–1000 cm�1 was collected using 32scans at a 4 cm�1 resolution for pyridine and 128 scans at 2 cm�1

resolution for DTBPy.

SSNMR spectroscopy was performed by using a Bruker Avance IIspectrometer with a 14.1 T wide-bore magnet using a 4 mm tripleresonance magic-angle spinning (MAS) probe in double resonancemode. Topspin 3.0 software was used for data acquisition andprocessing. The operating frequencies for 1H and 27Al on this spec-trometer are 600.13 and 156.38 MHz, respectively. The sampleswere first rehydrated in a humidifier for 48 h at RT. The powderswere then packed into a kel-F rotor insert and the insert wasplaced in a 4 mm MAS rotor. Samples were spun at a frequency of5 or 12 kHz, with the slower speed required for some samples ifspinning sidebands from the downfield peak interfered with the

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resonance of the upfield peak. The temperature was stabilized at298 K. Spectra were acquired using a 90–t–180–t–detect Hahnecho pulse sequence with a 2.5 ms 908 27Al pulse and an echoperiod of one rotor period (200 ms at 5 kHz spinning speed or83 ms at 12 kHz spinning) under 1H dipolar decoupling at 62 kHz.Typically, spectra were acquired with 2048 scans and a recycledelay of 1.5 s.

Catalyst testing

Catalytic pyrolysis experiments were conducted by using a micro-pyrolyzer (PY-2020iS, Frontier Laboratories, Japan) equipped withan autoshot sampler (AS-1020E, Frontier Laboratories, Japan). A de-tailed description of the setup can be found in previous stud-ies.[2b, 25] All CFP experiments were performed in situ. The zeolitecatalyst was mixed directly with biomass in a catalyst-to-biomassweight ratio of 20. Approximately 5 mg of biomass/catalyst mixturewas used in a typical experiment. He carrier gas was used tosweep the pyrolysis vapor into the GC (Varian CP3800, USA). Thevapor was separated in a GC capillary UA-1701 column. The GCoven was programmed for a 3 min hold at 40 8C followed by heat-ing (10 8C min�1) to 250 8C, after which temperature was held con-stant for 6 min. The injector temperature was 260 8C, and the injec-tor split ratio was set to 100:1. Separated pyrolysis vapors were an-alyzed either by a mass spectrometer detector (MSD) or a flameionization detector (FID). The MSD (Saturn 2200, Varian, USA) wasused for molecular identification. After the peaks were identified,standards were prepared to quantify the results using FID. Thefinal product distribution was reported as molar carbon yield, de-fined as the molar ratio of carbon in a specific product to thecarbon in the feedstock. Selectivity for aromatics in this study wasdefined as moles of carbon in a specific aromatic hydrocarbon tototal moles of carbon in the aromatic products.

Acknowledgements

This work was supported by the Iowa Energy Center under IECGrant Number 13-01. The authors would like to also thankWarren Straszheim and Scott Schlorholtz for their assistance withSEM and XRD investigations.

Keywords: aluminum · biomass · heterogeneous catalysis ·structure–activity relationships · zeolites

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Received: February 10, 2016Revised: March 23, 2016Published online on && &&, 0000

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T. C. Hoff, D. W. Gardner, R. Thilakaratne,K. Wang, T. W. Hansen, R. C. Brown,J.-P. Tessonnier*

&& –&&

Tailoring ZSM-5 Zeolites for the FastPyrolysis of Biomass to AromaticHydrocarbons

Playing nice: The interplay of zeolitestructural characteristics is studied toidentify parameters common to high-performance catalysts in catalytic fastpyrolysis. Crystallinity and accessibilityto framework Al atoms is critical to ach-ieve high aromatic yields. These findingsallow us to synthesize a ZSM-5 catalystwith enhanced catalytic properties,which offers the highest aromatic hy-drocarbon yield reported to date.

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