Millisecond Pulsed Films Unify the Mechanisms of Cellulose Fragmentation

The mechanism of crystalline cellulose fragmentation has been debated between classical models proposing end-chain or intrachain scission to form short-chain (molten) anhydro-oligomer mixtures and volatile organic compounds. Models developed over the last few decades suggest global kinetics consistent with either mechanism, but validation of the chain-scission mechanism via measured reaction rates of cellulose has remained elusive. To resolve these differences, we introduce a new thermal-pulsing reactor four orders of magnitude faster than conventional thermogravimetic analysis (10⁶ vs 10² °C/min) to measure the millisecond-resolved evolution of cellulose and its volatile products at 400–550 °C. By comparison of cellulose conversion and furan product formation kinetics, both mechanisms are shown to occur with the transition from chain-end scission to intrachain scission above 467 °C concurrent with liquid formation comprised of short-chain cellulose fragments

On the Yield of Levoglucosan from Cellulose Pyrolysis

Fast pyrolysis is a thermochemical process to fragment large biopolymers such as cellulose to chemical intermediates which can be refined to renewable fuels and chemicals. Levoglucosan (LGA), a six-carbon oxygenate, is the most abundant primary product from cellulose pyrolysis with LGA yields reported over a wide range of 5–80 percent carbon (%C). In this study, the variation of the observed yield of LGA from cellulose pyrolysis was experimentally investigated. Cellulose pyrolysis experiments were conducted in two different reactors: the Frontier micropyrolyzer (2020-iS), and the pulse heated analysis of solid reactions (PHASR) system. The reactor configuration and experimental conditions including cellulose sample size were found to have a significant effect on the yield of LGA. Four different hypotheses were proposed and tested to evaluate the relationship of cellulose sample size and the observed LGA yield including (a) thermal promotion of LGA formation, (b) the crystallinity of cellulose samples, (c) secondary and vapor-phase reactions of LGA, and (d) the catalytic effect of melt-phase hydroxyl groups. Co-pyrolysis experiments of cellulose and fructose in the PHASR reactor presented indirect experimental evidence of previously postulated catalytic effects of hydroxyl groups in glycosidic bond cleavage for LGA formation in transport-limited reactor systems

Silica Nanoparticle Mass Transfer Fins for MFI Composite Materials

Zeolite nanoparticles have been widely used to overcome diffusion limitations in heterogeneous catalytic reactions. However, the existence of surface barriers for molecular diffusion in zeolites can limit the benefits of using nanoparticles in catalytic reactions. In this study, a set of silica nanoparticle (SNP)/silicalite-1 composites with different external surface to micropore surface ratios was synthesized to understand the effects of surface-controlled mass transport on molecular diffusion in zeolite nanoparticles. The zero length column (ZLC) technique was used to evaluate the mass transport of cyclohexane in these materials. It was found that the strong sorbate/sorbent interaction at the external surface of silicalite-1 nanoparticles can cause diffusing molecules to re-enter into micropores and repeat the micropore diffusion process. This pore re-entry step can lead to an unusually long micropore diffusion length. We also demonstrated that this repeated micropore diffusion process can be effectively reduced by mixing the zeolite nanoparticles with secondary, nonporous nanoparticles. This study provides an alternative way to justify the surface mass transfer resistance, and it also introduces a simple strategy to enhance mass transport in zeolite nanoparticles other than surface modification which can damage the integrity of zeolite crystals. Additionally, previous diffusion results were revisited by adjusting the actual micropore diffusion length. It was concluded that the surface resistance in zeolite nanoparticles is likely due to a combination of pore re-entry of adsorbates and pore blockage

Long Walks in Hierarchical Porous Materials due to Combined Surface and Configurational Diffusion

Hierarchical materials with porous structures at different length scales (i.e., micropore and mesopore) are an emerging class of materials. However, the lack of fundamental understanding of mass transport properties significantly limits rational development of these materials for applications in catalysis and separation. In this study, we evaluated the mass transport of two probe molecules, cyclohexane and 1-methylnaphthalene, in two different types of hierarchical porous materials, SBA-15 mesoporous silica and three dimensionally ordered mesoporous imprinted (3DOm-i) silicalite-1 zeolite, for comparison with nonmicroporous MCM-41 mesoporous silica. It was observed that the apparent diffusion lengths determined for hierarchical porous materials (i.e., SBA-15 and 3DOm-i silicalite-1) were significantly longer than predicted by the physical structure (i.e., radius) of the adsorbent particle, indicating that diffusion of molecules in hierarchical porous materials is much longer than expected. The unusually long path length is likely due to diffusion on the external surface, followed by re-entering of diffusing molecules from the external surface into the micropores; the large external surface area of hierarchical porous materials enhances the extent of this phenomenon. The observations reported in the study highlight the importance of surface diffusion in hierarchical porous materials. Enhanced mass transport in hierarchical porous materials can be overpredicted without considering the extent of sorbate–sorbent interaction and the actual diffusion length

Ab Initio Dynamics of Cellulose Pyrolysis: Nascent Decomposition Pathways at 327 and 600 °C

We modeled nascent decomposition processes in cellulose pyrolysis at 327 and 600 °C using Car–Parrinello molecular dynamics (CPMD) simulations with rare events accelerated with the metadynamics method. We used a simulation cell comprised of two unit cells of cellulose Iβ periodically repeated in three dimensions to mimic the solid cellulose. To obtain initial conditions at reasonable densities, we extracted coordinates from larger classical NPT simulations at the target temperatures. CPMD-metadynamics implemented with various sets of collective variables, such as coordination numbers of the glycosidic oxygen, yielded a variety of chemical reactions such as depolymerization, fragmentation, ring opening, and ring contraction. These reactions yielded precursors to levoglucosan (LGA)the major product of pyrolysisand also to minor products such as 5-hydroxy-methylfurfural (HMF) and formic acid. At 327 °C, we found that depolymerization via ring contraction of the glucopyranose ring to the glucofuranose ring occurs with the lowest free-energy barrier (20 kcal/mol). We suggest that this process is key for formation of liquid intermediate cellulose, observed experimentally above 260 °C. At 600 °C, we found that a precursor to LGA (pre-LGA) forms with a free-energy barrier of 36 kcal/mol via an intermediate/transition state stabilized by anchimeric assistance and hydrogen bonding. Conformational freedom provided by expansion of the cellulose matrix at 600 °C was found to be crucial for formation of pre-LGA. We performed several comparison calculations to gauge the accuracy of CPMD-metadynamics barriers with respect to basis set and level of theory. We found that free-energy barriers at 600 °C are in the order pre-LGA < pre-HMF < formic acid, explaining why LGA is the kinetically favored product of fast cellulose pyrolysis

Enhanced Molecular Transport in Hierarchical Silicalite‑1

Fundamental understanding of the mass transport of petrochemical and biomass derived molecules in microporous and mesoporous solid catalysts is important for developing the next generation of heterogeneous catalysts for traditional hydrocarbon processing including biomass pyrolysis and upgrading. Hierarchical zeolites with both micropores and mesopores exhibit enhanced mass transport and unique catalytic performance in reactions involving large molecules. However, quantitative description of mass transport in such materials remains elusive, owing to the complicated structure of hierarchical pores and difficulty in the synthesis of the materials with controllable structures. In this work, zero length column chromatography (ZLC) was used to study temperature-dependent diffusion of cyclohexane in silicalite-1, self-pillared pentasil (SPP) zeolite, and three-dimensionally ordered mesoporous imprinted (3DOm-i) silicalite-1. The samples were synthesized with controllable characteristic diffusion lengths from micrometer scale (ca. 20 μm) to nanometer scale (ca. 2 nm), allowing systematic study of the effect of mesoporosity on the mass transport behavior of hierarchical zeolites. The results show that the introduction of mesoporosity can indeed significantly facilitate the mass transport of cyclohexane in hierarchical silicalite-1 by reducing diffusional time constants, indicating rapid overall adsorption and desorption. However, when the length scale of the material approaches several nanometers, the contribution from the surface resistance, or “surface barrier”, to overall mass transfer becomes dominant

Understanding Diffusion in Hierarchical Zeolites with House-of-Cards Nanosheets

Introducing mesoporosity to conventional microporous sorbents or catalysts is often proposed as a solution to enhance their mass transport rates. Here, we show that diffusion in these hierarchical materials is more complex and exhibits non-monotonic dependence on sorbate loading. Our atomistic simulations of <i>n</i>-hexane in a model system containing microporous nanosheets and mesopore channels indicate that diffusivity can be smaller than in a conventional zeolite with the same micropore structure, and this observation holds true even if we confine the analysis to molecules completely inside the microporous nanosheets. Only at high sorbate loadings or elevated temperatures, when the mesopores begin to be sufficiently populated, does the overall diffusion in the hierarchical material exceed that in conventional microporous zeolites. Our model system is free of structural defects, such as pore blocking or surface disorder, that are typically invoked to explain slower-than-expected diffusion phenomena in experimental measurements. Examination of free energy profiles and visualization of molecular diffusion pathways demonstrates that the large free energy cost (mostly enthalpic in origin) for escaping from the microporous region into the mesopores leads to more tortuous diffusion paths and causes this unusual transport behavior in hierarchical nanoporous materials. This knowledge allows us to re-examine zero-length-column chromatography data and show that these experimental measurements are consistent with the simulation data when the crystallite size instead of the nanosheet thickness is used for the nominal diffusional length

Five Rules for Measuring Biomass Pyrolysis Rates: Pulse-Heated Analysis of Solid Reaction Kinetics of Lignocellulosic Biomass

Pyrolytic conversion of lignocellulosic biomass utilizes high temperatures to thermally fragment biopolymers to volatile organic compounds. The complexity of the degradation process includes thousands of reactions through multiple phases occurring in less than a second. In this work, the requirements are established for measuring the reaction kinetics of high temperature (>400 °C) biomass pyrolysis in the absence of heat and mass transfer limitations. Additionally, experimental techniques must heat and cool biomass samples sufficiently fast to elucidate the evolution of reaction products with time while also eliminating a substantial reaction during the heating and cooling phases, preferably by measuring the temperature of the reacting biomass sample directly. These requirements are described with the PHASR (pulse-heated analysis of solid reactions) technique and demonstrated by measuring the time-resolved evolution of six major chemical products from loblolly pine pyrolysis over a temperature range of 400 to 500 °C. Differential kinetics of loblolly pine pyrolysis are measured to determine the apparent activation energy for the formation of six major product compounds including levoglucosan, furfural, and 2-methoxyphenol

ReEngineered Feedstocks for Pulverized Coal Combustion Emissions Control

New coal reaction technology called ReEngineered Feedstock (ReEF), consisting of post-recycled paper and plastics, was evaluated for combustion emissions control when cofiring with pulverized coal. Experiments were conducted with four types of ReEF in a 2 in. diameter laboratory-scale fluidized bed combustor system heated to 1200, 1400, and 1600 °C. Flue gas emission was continuously monitored with an online infrared multigas analyzer and mass spectrometer. The results indicate that co-firing ReEF with coal provides SO₂ emission reduction in flue gas up to 85% and moderate decrease in NO emissions, as well as higher carbon conversion than pure coal combustion. ReEF, slag and fly ash solids were were analyzed by X-ray diffraction; identification of sulfates in the product ash conclusively supports the mechanism of in situ sulfur capture

Effect of Temperature and Transport on the Yield and Composition of Pyrolysis-Derived Bio-Oil from Glucose

The fast pyrolysis of biomass forms bio-oil, char, and light noncondensable gases. Bio-oil is the desired product in context of converting biomass to biofuel. The effect of temperature on bio-oil yield and composition is anticipated to be different under reaction-limited and transport-limited operating conditions. Attaining fundamental understanding of the effect of temperature and transport on bio-oil yield and composition is challenging, because of limited knowledge of pyrolysis chemistry and the inter-relationship between chemistry and transport. In this work, we performed thin-film and powder pyrolysis experiments to investigate the thermal decomposition of glucose (biomass model compound) under both reaction-controlled and transport-limited operating conditions. In thin-film (size ≤10 μm) experiments, the effect of temperature on pyrolysis product distribution, especially on bio-oil yield and composition, was studied. In addition, using the thin-film data, mechanistic insights into glucose decomposition were provided and a map of reaction pathways was proposed. Decomposition of glucose in the reaction-controlled regime is initiated by dehydration reactions. With increase in temperature, anhydrosugars (viz, levoglucosan and levoglucosenone) apparently converted to furans (hydroxymethylfurfural) and light oxygenates (formic acid/methyl glyoxal), respectively, as ring opening and fragmentation reactions became more facile. Pyrans remained relatively stable. The effect of transport was investigated by performing pyrolysis experiments with different particle sizes. The variation in the yield and composition of bio-oil, with respect to temperature and particle size, was also analyzed. In the case of glucose powder, levoglucosan yield increased significantly with particle size but decreased marginally with temperature, while hydroxymethylfurfural, furfural, formic acid, and methyl glyoxal yields monotonically increased as the temperature and particle size each increased. A thin film of glucose gave a lower yield of bio-oil and a higher yield of char than that of glucose powder