Steven Crossley

Understanding the role of TiO2 crystal structure on the enhanced activity and stability of Ru/TiO2 catalysts for the conversion of lignin-derived oxygenates

Although Ru catalysts supported on reducible oxides such as TiO2 hold significant promise for the deoxygenation of biomass derived oxygenates, a significant drawback is their instability under oxidation conditions necessary for catalyst regeneration. In this contribution, the role of TiO2 crystal structure on resistance to metal particle sintering during calcination treatments at 400 and 500 °C is investigated. The resulting impact of the calcination temperature and TiO2 support phase for the conversion of guaiacol at 400 °C under atmospheric pressure of hydrogen over supported Ru catalysts is presented. Results suggest that the rutile TiO2 phase plays an important role in stabilizing Ru particles during calcination pretreatment in comparison with anatase supported Ru catalysts. Furthermore, rates normalized to the area of the support and the Ru suggest that the high activity of Ru/TiO2 systems for guaiacol conversion is attributed to defect sites created by hydrogen spillover from the Ru metal to the reducible TiO2 as opposed to only the sites located at the Ru/TiO2 interface.

Epitaxial Growth of ZSM-5@Silicalite-1: A Core–Shell Zeolite Designed with Passivated Surface Acidity

The design of materials with spatially controlled chemical composition has potential advantages for wide-reaching applications that span energy to medicine. Here, we present a method for preparing a core-shell aluminosilicate zeolite with continuous translational symmetry of nanopores and an epitaxial shell of tunable thickness that passivates Brønsted acid sites associated with framework Al on exterior surfaces. For this study, we selected the commercially relevant MFI framework type and prepared core-shell particles consisting of an aluminosilicate core (ZSM-5) and a siliceous shell (silicalite-1). Transmission electron microscopy and gas adsorption studies confirmed that silicalite-1 forms an epitaxial layer on ZSM-5 crystals without blocking pore openings. Scanning electron microscopy and dynamic light scattering were used in combination to confirm that the shell thickness can be tailored with nanometer resolution (e.g., 5-20 nm). X-ray photoelectron spectroscopy and temperature-programmed desorption measurements revealed the presence of a siliceous shell, while probe reactions using molecules that were either too large or adequately sized to access MFI pores confirmed the uniform shell coverage. The synthesis of ZSM-5@silicalite-1 offers a pathway for tailoring the physicochemical properties of MFI-type materials, notably in the area of catalysis, where surface passivation can enhance product selectivity without sacrificing catalyst activity. The method described herein may prove to be a general platform for zeolite core-shell design with potentially broader applicability to other porous materials.

Zeolite-catalysed C–C bond forming reactions for biomass conversion to fuels and chemicals

Biomass conversion to fuels requires elimination of oxygenated functionalities along with formation of C–C bonds to help keeping the largest possible amount of carbon in the fuel range (e.g. C7–C15). Acidic zeolites are particularly effective in catalysing C–C bond forming reactions from molecules containing oxygenated functionalities such as carbonyl, carboxylic, and hydroxide groups, including acylation, ketonization, and alkylation. The nature of the active sites and possible reaction mechanisms for these reactions are discussed in this mini-review.

Decoupling HZSM‐5 Catalyst Activity from Deactivation during Upgrading of Pyrolysis Oil Vapors

The independent evaluation of catalyst activity and stability during the catalytic pyrolysis of biomass is challenging because of the nature of the reaction system and rapid catalyst deactivation that force the use of excess catalyst. In this contribution we use a modified pyroprobe system in which pulses of pyrolysis vapors are converted over a series of HZSM-5 catalysts in a separate fixed-bed reactor controlled independently. Both the reactor-bed temperature and the Si/Al ratio of the zeolite are varied to evaluate catalyst activity and deactivation rates independently both on a constant surface area and constant acid site basis. Results show that there is an optimum catalyst-bed temperature for the production of aromatics, above which the production of light gases increases and that of aromatics decrease. Zeolites with lower Si/Al ratios give comparable initial rates for aromatics production, but far more rapid catalyst deactivation rates than those with higher Si/Al ratios.

Gluconic Acid from Biomass Fast Pyrolysis Oils: Specialty Chemicals from the Thermochemical Conversion of Biomass

Fast pyrolysis of biomass to produce a bio-oil followed by catalytic upgrading is a widely studied approach for the potential production of fuels from biomass. Because of the complexity of the bio-oil, most upgrading strategies focus on removing oxygen from the entire mixture to produce fuels. Here we report a novel method for the production of the specialty chemical, gluconic acid, from the pyrolysis of biomass. Through a combination of sequential condensation of pyrolysis vapors and water extraction, a solution rich in levoglucosan is obtained that accounts for over 30% of the carbon in the bio-oil produced from red oak. A simple filtration step yields a stream of high-purity levoglucosan. This stream of levoglucosan is then hydrolyzed and partially oxidized to yield gluconic acid with high purity and selectivity. This combination of cost-effective pyrolysis coupled with simple separation and upgrading could enable a variety of new product markets for chemicals from biomass.

Multiwalled Carbon Nanotubes at the Interface of Pickering Emulsions

Carbon nanotubes exhibit very unique properties in biphasic systems. Their interparticle attraction leads to reduced droplet coalescence rates and corresponding improvements in emulsion stability. Here we use covalent and noncovalent techniques to modify the hydrophilicity of multiwalled carbon nanotubes (MWCNTs) and study their resulting behavior at an oil-water interface. By using both paraffin wax/water and dodecane/water systems, the thickness of the layer of MWNTs at the interface and resulting emulsion stability are shown to vary significantly with the approach used to modify the MWNTs. Increased hydrophilicity of the MWNTs shifts the emulsions from water-in-oil to oil-in-water. The stability of the emulsion is found to correlate with the thickness of nanotubes populating the oil-water interface and relative strength of the carbon nanotube network. The addition of a surfactant decreases the thickness of nanotubes at the interface and enhances the overall interfacial area stabilized at the expense of increased droplet coalescence rates. To the best of our knowledge, this is the first time the interfacial thickness of modified carbon nanotubes has been quantified and correlated to emulsion stability.

Catalytic strategies for improving specific fuel properties

As governments are urged to become more diligent in tackling environmental issues, legislatures in many countries continue to set increasingly stringent environmental standards on petroleum fuels, especially in reference to sulfur and polynuclear aromatic hydrocarbon (PAH or PNA) contents, which in ...

Multistage torrefaction and in situ catalytic upgrading to hydrocarbon biofuels: analysis of life cycle energy use and greenhouse gas emissions

A well-to-wheel life cycle assessment (LCA) model is developed to characterize the life cycle energy consumption and greenhouse gas emissions profiles of a series of novel multistage torrefaction and pyrolysis systems for targeted thermochemical conversion of short rotation woody crops to bio-oil and in situ catalytic upgrading to hydrocarbon transportation fuels, and to benchmark the results against a base-case fast pyrolysis and hydrodeoxygenation (HDO) platform. Multistage systems utilize a staged thermal gradient to fractionate bio-oil into product streams consisting of distinct functional groups, and multi-step chemical synthesis for downstream processing of bio-oil fractions to hydrocarbon fuels. Results at the process scale reveal that multistage systems have several advantages over the base-case including: (1) ∼40% reduction in process hydrogen consumption and (2) the product distribution for multistage systems are skewed towards longer carbon chain compounds that are fungible with diesel-range fuels. LCA reveals that the median Energy Return On Investment (EROI) and life cycle greenhouse gas (GHG) emissions for multistage systems range from 1.32 to 3.76 MJ-fuel/MJ-primary fossil energy and 17.1 to 52.8 gCO2e/MJ-fuel respectively, over the host of co-product scenarios and allocation schemes analyzed, with fossil-derived hydrogen constituting the principle GHG and primary energy burden across all systems. These results are compelling and indicate that multistage systems exhibit comparatively higher gasoline/diesel-range fuel yield relative to current technology, and produce a high quality infrastructure-compatible hydrocarbon transportation fuel capable of achieving over 80% reduction in life cycle GHG emissions relative to baseline petroleum diesel.

Direct carbon-carbon coupling of furanics with acetic acid over Brønsted zeolites.

Zeolite-catalyzed acylation is used for selective formation of C–C bonds and could have applications in bioenergy. Effective carbon-carbon coupling of acetic acid to form larger products while minimizing CO2 emissions is critical to achieving a step change in efficiency for the production of transportation fuels from sustainable biomass. We report the direct acylation of methylfuran with acetic acid in the presence of water, all of which can be readily produced from biomass. This direct coupling limits unwanted polymerization of furanics while producing acetyl methylfuran. Reaction kinetics and density functional theory calculations illustrate that the calculated apparent barrier for the dehydration of the acid to form surface acyl species is similar to the experimentally measured barrier, implying that this step plays a significant role in determining the net reaction rate. Water inhibits the overall rate, but selectivity to acylated products is not affected. We show that furanic species effectively stabilize the charge of the transition state, therefore lowering the overall activation barrier. These results demonstrate a promising new route to C–C bond–forming reactions for the production of higher-value products from biomass.

Enhancing the Acylation Activity of Acetic Acid by Formation of an Intermediate Aromatic Ester

Acylation is an effective C-C bond-forming reaction to condense acetic acid and lignin-derived aromatic compounds into acetophenones, valuable precursors to fuels and chemicals. However, acetic acid is intrinsically an ineffective acylating agent. Here, we report that its acylation activity can be greatly enhanced by forming intermediate aromatic esters directly derived from acetic acid and phenolic compounds. Additionally, the acylation reaction was studied in the liquid phase over acid zeolites and was found to happen in two steps: 1) formation of an acylium ion and 2) C-C bond formation between the acylium ion and the aromatic substrate. Each of these steps may be rate-limiting, depending on the type of acylating agent and the aromatic substrate. Oxygen-containing substituents, such as -OH and -OCH3 , can activate aromatic substrates for step 2, with -OH> -OCH3 , whereas alkyl substituent -R cannot. At the same time, aromatic esters can rearrange to acetophenones by both an intramolecular pathway and, preferentially, an intermolecular one.