Article pubs.acs.org/JPCC
Catalytic Upgrading of Biomass-Derived Compounds via C−C Coupling Reactions: Computational and Experimental Studies of Acetaldehyde and Furan Reactions in HZSM‑5 Cong Liu,† Tabitha J. Evans,‡ Lei Cheng,† Mark R. Nimlos,‡ Calvin Mukarakate,‡ David J. Robichaud,‡ Rajeev S. Assary,*,† and Larry A. Curtiss*,† †
Materials Sciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States
‡
S Supporting Information *
ABSTRACT: Catalytic C−C coupling and deoxygenation reactions are essential for upgrading of biomass-derived oxygenates to fuel-range hydrocarbons. Detailed understanding of mechanistic and energetic aspects of these reactions is crucial to enabling and improving the catalytic upgrading of small oxygenates to useful chemicals and fuels. Using periodic density functional theory (DFT) calculations, we have investigated the reactions of furan and acetaldehyde in an HZSM-5 zeolite catalyst, a representative system associated with the catalytic upgrading of pyrolysis vapors. Comprehensive energy profiles were computed for self-reactions (i.e., acetaldehyde coupling and furan coupling) and cross-reactions (i.e., acetaldehyde + furan) of this representative mixture. Major products proposed from the computations are further confirmed using temperature controlled mass spectra measurements. The computational results show that furan interacts with acetaldehyde in HZSM-5 via an alkylation mechanism, which is more favorable than the self-reactions, indicating that mixing furans with aldehydes could be a promising approach to maximize effective C−C coupling and dehydration while reducing the catalyst deactivation (e.g., coke formation) from aldehyde condensation.
1. INTRODUCTION
possibly utilizing these oxygenates to obtain longer chain hydrocarbons are necessary for bio-oil upgrading. Starting from biomass derived compounds with C1−C6 carbons, catalytically controlled C−C coupling and deoxygenation are effective postpyrolysis treatments for the conversion of bio-oil to transportation fuels such as gasoline (C4−C12), jet fuels (C9−C16), and diesel fuels (C9−C20).8,27,38,39 Zeolites offer a promising class of heterogeneous catalysts for the petrochemical industry and are likely to play a crucial role in the (industry scale) catalytic conversion of biomass.6,11,20,21,27,32,37 Among various zeolite materials, HZSM5 has gained increased attention since the development of the methanol-to-gasoline (MTG) process5 due to its outstanding catalytic properties. Numerous studies have been reported utilizing the catalytic properties of zeolites for biomass upgrading, and only selected studies are pointed out here. For example, Bakhshi and co-workers22,40 have studied upgrading of wood-derived bio-oil using different catalysts, including HZSM-5, HY, mordenite, silicalite-1, and alumina−
Declining fossil fuels with increasing demand for energy, as well as environmental concerns of greenhouse gas emissions, have led to the exploration of utilizing biomass materials as alternative renewable carbon resources.8,19 Increasing effort has been focused on developing effective processes to convert biomass materials to liquid fuels.8,9,19,38 Current techniques for biomass conversion include thermochemical (liquefaction, fast pyrolysis, catalytic fast pyrolysis, and alcohol/oxygenate synthesis from syngas) and biochemical processes (digestion and fermentation).29,30,44 Among these techniques, fast pyrolysis with short vapor residence times (less than 2 s), rapid heating rates (up to 104 °C·s−1), and moderate temperatures ( ethyl acetate. The aldol condensation product of acetaldehyde, crotonaldehyde (shown in Figure 1), can undergo condensation and elimination to form benzene, which results in coke formation.37 Based on these detailed studies of different oxygenated compounds in zeolites, it is clear that the chemistry and optimal reaction condition of the catalytic conversion of these compounds depend not only on the catalytic and structural properties of zeolite catalysts, but also on the functional groups of the reactants. Although optimizing the reaction condition of certain model compounds is relatively straightforward, it is challenging to control the catalytic reactions of a mixture of various oxygenated compounds. The above studies14−16,23−35 have suggested that certain functional groups (e.g., aldehydes) are more likely to deactivate zeolites via coke formation than others. Then a few interesting questions can be raised: Are there any effective reactions that could convert aldehydes to other, more manageable oxygenates? How could we control the reaction condition to favor the effective conversion of aldehydes other than the deactivation mechanism? Although several researchers15,37 have reported the deactivation of HZSM-5 by aldehydes, there is a paucity of research on (a) how aldehydes interact with other biomass derived compounds or (b) detailed reaction mechanisms and the thermochemistry of aldehyde related reactions in zeolite catalysts. Our previous study using highly accurate computational approaches (e.g., G4 method) has mapped the thermochemistry associated with the gas phase coupling reactions of furan with various C1−C4 oxygenated compounds (e.g., aldehydes, alkenes, and furans) to produce C5−C8 cyclic ethers.28 The calculated apparent barriers of these reactions (∼1 eV) are lower than those of the cellulose activation or decomposition reactions (>1.5 eV).51 This indicates that furan could be an appropriate diene for C−C coupling reactions with biomass derived oxygenates. In the present work, we report the catalytic reactions of furan and acetaldehyde in HZSM-5 as a representative system. Periodic density functional theory (DFT) calculations were carried out, utilizing a realistic catalyst model, to study detailed catalytic reaction pathways for selfreactions (acetaldehyde coupling and furan coupling) and cross-reactions (acetaldehyde + furan). A schematic representation of these reactions is shown in Figure 1. Temperature controlled vapor-phase experiments were also performed to verify the reaction products from the DFT results. The complementary experiments support the computational results, which has given insights into rate-determining steps for the selfand cross-reactions of furan and aldehyde types of oxygenated compounds.
silica. HZSM-5 was shown to give the highest organic fraction (34 wt %) relative to the bio-oil feed, compared to the other tested catalysts. Besides the high activity, HZSM-5 has the ideal pore size (6−7 Å) to upgrade bio-oil to a hydrocarbon fraction which resembles gasoline. Also, optimal internal pore space and shape selectivity of HZSM-5 promote the formation of aromatic compounds. Recently, Huber and co-workers3,4,21 investigated numerous catalysts for bio-oil upgrading, and concluded that HZSM-5 gave the highest yield of aromatics. In addition, HZSM-5 has shown slow deactivation by coke formation from alcohol functional group and good hydrothermal stability.37 The catalytic C−C coupling reactions and deoxygenation associated with the upgrading of mixed low molecular weight oxygenates are rather complicated. Often, model compounds are studied both experimentally and computationally to help better understand the molecular pathways of the reactions taking place in zeolites (refs 2, 11, 14−16, 20, 34, 35, 37, 42, 43, 46−50). One example is zeolite catalyzed conversion of methanol to hydrocarbons (MTH) reported by Ilias and Bhan.20 They have described major chemical processes associated with MTH such as methylation, olefin cracking, hydrogen transfer, cyclization, aromatic methylation, and aromatic dealkylation and concluded that the product distributions of MTH are affected by zeolite topology and operating conditions.20 In addition to methanol related reactions, investigations have been also carried out on other oxygenated species. Recently, furans derived from biomass have received increasing attention in catalytic biomass conversion to fuels.6,11,28,46,49 Huber and co-workers have reported that furan forms aromatics and olefins in HZSM-5, in which benzofuran, as the major furan coupling product (see Figure 1), is an important intermediate for the formation of benzene and alkenes.6,7 In addition, Williams et al.49 have reported a “green” route to convert 2,5-dimethylfuran (DMF) and ethylene to pxylene in HY zeolite with a 75% selectivity at 300 °C. The reaction occurs via Diels−Alder reaction followed by dehydration reaction.49 Gayubo et al.14 demonstrated that alcohols dehydrate rapidly to corresponding olefins in HZSM-5 at low temperature (around 200 °C). Iso-alcohols dehydrate more rapidly than linear alcohols. At higher temperatures (>350 °C), the olefins are transformed into C4+ paraffins and aromatics. On the other hand, phenol tends to have a low reactivity on HZSM-5 and produces small amounts of olefins. Moreover, the rate of deactivation by coke formation is low for the transformation of alcohols and phenols and decreases as the water content is increased. More recently, Ramasamy et al.37 investigated the HZSM-5 deactivation using aqueous feed mixtures of selected oxygenates at 360 °C, suggesting that the deactivation rate of HZSM-5 is in the order acetaldehyde > 24026
DOI: 10.1021/acs.jpcc.5b08141 J. Phys. Chem. C 2015, 119, 24025−24035
Article
The Journal of Physical Chemistry C
Figure 2. Unit cell structure of the computational model of HZSM-5. (a), (b), and (c) represent the perspectives along the x, y, and z axes, respectively.
2. METHODS 2.1. Computational Section. All the calculations were performed using the Vienna Ab initio Simulation Package (VASP, version 5.3).23−26 The unit cell was constructed from the full siliceous crystallographic structure of ZSM-5; one Si atom in the unit cell was replaced by an Al atom, and a proton was added to one of the Al−O−Si sites to balance the charge and create an acidic site. Since the computational study is focused on the reactivity of the catalyst, only one active site was built in the unit cell, and the Si/Al atom ratio is 95. The unit cell used for the computations is shown in Figure 2 (perspectives along x, y, and z axes are also shown). The constructed unit cell of HZSM-5 consists of 298 atoms, all of which plus the molecular compounds were allowed to relax during the geometry optimization, while the lattice parameters of a full siliceous ZSM-5 were used for the unit cell and kept constant. The gas-phase molecules were placed in a 15 × 15 × 15 Å vacuum box, and were allowed to fully relax during the optimization. (Details regarding the computational resources utilized for the calculations are presented in the Supporting Information.) The PBE functional36 with a plane wave basis set was applied, with a cutoff energy of 400 eV. The Γ-point and the 2 × 2 × 1 k-point mesh were used to sample the Brillouin zones of the gas-phase molecules and zeolite-phase calculations, respectively. A quasi-Newton force-minimization algorithm was employed for all the calculations. Convergence was assumed to be achieved when forces were lower than 0.05 eV/Å. Transition states were searched through the climbing-image nudged elastic band (CI-NEB) method.17,31,45 Four to six images were defined along the investigated reaction pathway. Each image was optimized by allowing a relaxation in the subspace perpendicular to the reaction coordinate, using the same functional and convergence criteria as above. Transition states were further verified through frequency calculations. During the investigations of the reaction barriers, the proton transfer barriers during dehydration and protonation of acetaldehyde were found to be very small (
