Research Article pubs.acs.org/journal/ascecg
Deactivation of Multilayered MFI Nanosheet Zeolite during Upgrading of Biomass Pyrolysis Vapors Mengze Xu,†,‡ Calvin Mukarakate,*,‡ Kristiina Iisa,‡ Sridhar Budhi,†,‡ Martin Menart,† Malcolm Davidson,† David J. Robichaud,‡ Mark R. Nimlos,‡ Brian G. Trewyn,† and Ryan M. Richards*,†,‡ †
Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States National Renewable Energy Laboratory, 15523 Denver West Parkway, Golden, Colorado 80401, United States
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S Supporting Information *
ABSTRACT: The catalytic fast pyrolysis (CFP) of biomass is a promising technology for producing renewable transportation fuels and chemicals. MFI-type catalysts have shown promise for CFP because they produce gasoline range hydrocarbons from oxygenated pyrolysis compounds; however, rapid catalyst deactivation due to coking is one of the major technical barriers inhibiting the commercialization of this technology. Coke deposited on the surface of the catalysts blocks access to active sites in the micropores leading to rapid catalyst deactivation. Our strategy is to minimize rapid catalyst deactivation by adding mesoporosity through formation of MFI nanosheet materials. The synthesized MFI nanosheet catalysts were fully characterized and evaluated for cellulose pyrolysis vapor upgrading to produce olefins and aromatic hydrocarbons. The data obtained from pyrolysis-GCMS (py-GCMS) showed that fresh MFI nanosheets produced similar aromatic hydrocarbon and olefin yields compared to those of conventional HZSM-5. However, MFI nanosheets demonstrated a longer lifetime than HZSM-5 even though coke contents were also higher than those for HZSM-5 because the mesopores enabled better accessibility to active acid sites. This conclusion was supported by results from postreaction analysis of various spent catalysts collected at different points during the deactivation experiments. KEYWORDS: HZSM-5 deactivation, Mesoporous zeolite, Catalytic fast pyrolysis, Coke formation, Mesoporosity, Zeolite acidity
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pyrolysis vapors are bigger than 5 Å.8 The slow diffusion of these molecules can lead to additional molecular weight growth (or coking) which can in turn block external acid sites and obstruct the channels to internal acid sites in micropores, resulting in rapid catalyst deactivation. Mesoporous (>2 nm) materials such as MCM-41 and SBA-15 have been used for CFP of biomass and bio-oils to overcome the diffusion limitation of microporous zeolites.9−13 However, these materials are not effective at forming aromatic hydrocarbons because they have weaker acid sites and fewer Brønsted acid sites compared to those in HZSM-5 as well as exhibiting poor hydrothermal stability. Thus, the mesoporous systems only lead to partial deoxygenation of biomass vapors, enhancing the formation of furans and phenol,9−13 and also increasing coke formation.6 Modifying the zeolite structure by introducing secondary porosity is an approach that can overcome the molecular diffusion limitations, without altering the MFI Brønsted acid strengths, and thus reduce rapid deactivation of the catalyst
INTRODUCTION The production of fuels and chemicals from biomass can lead to the development of local economies and jobs and improve countries’ energy security. Therefore, significant research efforts are underway for utilizing biomass as a partial replacement of fossil feedstocks.1−3 Fast pyrolysis of biomass has attracted substantial attention because it can produce up to 75 wt % yield of bio-oil. However, the produced bio-oil has several undesirable characteristics such as high acidity, low heating value, immiscibility with hydrocarbons, and instability.4,5 Ex-situ catalytic fast pyrolysis (ex-situ CFP) is an attractive technique that combines fast pyrolysis with catalytic vapor phase upgrading to improve the quality of the bio-oil. Zeolites, especially HZSM-5 (MFI), are widely used catalysts for catalytic fast pyrolysis (CFP), due to their strong Brønsted acidity, high surface area, and unique microporous topologies.4,6,7 Although HZSM-5 is effective at deoxygenating biomass pyrolysis products to form olefins and aromatic hydrocarbons, the yields are typically low, and rapid deactivation is problematic.4,7 The micropores in the zeolite structure are hypothesized to greatly hinder the diffusion of bulky molecules in and out of the catalyst during biomass CFP because the kinetic diameters of some molecules in biomass © 2017 American Chemical Society
Received: March 16, 2017 Revised: April 21, 2017 Published: May 2, 2017 5477
DOI: 10.1021/acssuschemeng.7b00817 ACS Sustainable Chem. Eng. 2017, 5, 5477−5484
Research Article
ACS Sustainable Chemistry & Engineering without greatly changing the product distribution.14,15 Specifically, hierarchical mesoporous zeolites synthesized via templates allow the generation of regular mesopores and the flexibility of tuning pore sizes.16−18 These materials have been mainly used to study model compounds such as methanol.7 Ultrathin MFI nanosheet (2 nm thick) catalysts were reported by Choi and co-workers, and demonstrated that the nanosheets greatly increased the catalyst lifetime during methanol to gasoline (MTG) reactions, due to the introduced mesoporosity and additional acid sites generated on the mesopore surfaces.19 There have been limited applications of mesoporous zeolites to the catalytic upgrading of biomass vapors and biooils.16,20−22 Kelkar and co-workers prepared mesoporous MSU-MFI catalysts and applied them to the catalytic upgrading of poplar pyrolysis vapors.16 The MSU-MFI catalysts produced comparable hydrocarbon yields to HZSM-5, with improved selectivity toward C8 and C9 monoaromatics. Gamliel and coworkers evaluated a series of mesoporous MFI-type zeolite catalysts synthesized using both top-down and bottom-up approaches for the catalytic pyrolysis of cellulose and miscanthus.21 They determined that a higher yield of hydrocarbons can be obtained from catalysts with both optimum pore size and acidity. A larger mesopore volume causes polymerization of aromatics to form coke. Hoff and coworkers reported the application of a highly crystalline zeolite on CFP of cellulose and reported improved hydrocarbon yields relative to commercial HZSM-5.22 The higher yields were due to optimization of crystallinity and accessibility to framework aluminum sites. Park and co-workers studied the impact of unilamellar MFI nanosheets with a very large pore size of 6.8 nm for bio-oil upgrading and compared the results with those obtained using a mesoporous catalyst (SBA-15).20 The MFI nanosheet improved the properties of the bio-oil by reducing the amount of oxygenates and increasing the amount of aromatics due to the stronger Brønsted acid sites compared to SBA-15. In these studies, biomass samples and catalysts were mixed together (biomass-to-catalyst ratios
