Helen Y. Luo

Domino Reaction Catalyzed by Zeolites with Brønsted and Lewis Acid Sites for the Production of γ-Valerolactone from Furfural†

The development of more carbon efficient and economically viable lignocellulosic biomass conversion technologies is critical for the sustainable production of liquid transportation fuels and chemicals. The molecule g-valerolactone (GVL) has gained attention as a versatile platform chemical for the production of liquid alkenes, as a solvent for biomass processing, as an approved fuel additive, and as a precursor for renewable polymers. Biomass-derived GVL is currently produced by the multistep processing of the carbohydrate fractions of lignocelluloses, wherein acid catalysts transform sugars into levulinic acid (LA), and noble-metal catalysts reduce LA to GVL with molecular hydrogen (H2). [5,6] This strategy suffers from several limitations that hinder the large-scale manufacture of GVL. In particular, the LA-reduction step necessitates precious-metal catalysts (e.g., Ru or Pt) or high H2 pressures (> 30 bar), which have been shown to negatively impact the economics of GVL-derived transportation fuels. Formic acid has emerged as an alternative to molecular H2, but noble metals and/or harsh conditions are still required to carry out the hydrogenation step. Inexpensive supported transition metals (e.g., Cu/ Al2O3) are active but suffer from leaching and/or sintering during the reaction. For the large-scale production of GVL, catalytic schemes are required that maximize product yields without the use of precious metals, high H2 pressure, or an excessive number of unit operations. Transfer-hydrogenation (TH) reactions, such as the Meerwein–Ponndorf–Verley (MPV) reaction, offer an attractive alternative to molecular H2 for the reduction of targeted functional groups. The interaction between the catalyst, the hydrogen donor, and the acceptor molecule can be modulated to impact activity and selectivity. Many catalysts are active for TH reactions, including organometallic compounds, transition metals, and metal oxides featuring acid/base properties. Pure-silica zeolites containing a small amount of tetravalent heteroatoms with open coordination sites (e.g., Zr or Sn) have also been used as solid Lewis acids to promote TH reactions. Wise and Williams used homogeneous Ru complexes and Chia and Dumesic used heterogeneous metal oxides to produce GVL from levulinate derivatives by TH reactions with high yields. Corma and co-workers demonstrated the efficacy of Sn-Beta and Zr-Beta zeolites for the intermolecular MPV reaction between alcohols and ketones in organic solvents. Sn-Beta and other tin-containing silicates have also been shown to transform hexoses, pentoses, and trioses through intramolecular hydride and carbon-atom shifts in both organic and aqueous media. Herein, we report an integrated catalytic process for the efficient production of GVL from furfural (Fur) through sequential TH and hydrolysis reactions catalyzed by zeolites with Bronsted and Lewis acid sites (Scheme 1). In our

SSZ-13 Crystallization by Particle Attachment and Deterministic Pathways to Crystal Size Control

Many synthetic and natural crystalline materials are either known or postulated to grow via nonclassical pathways involving the initial self-assembly of precursors that serve as putative growth units for crystallization. Elucidating the pathway(s) by which precursors attach to crystal surfaces and structurally rearrange (postattachment) to incorporate into the underlying crystalline lattice is an active and expanding area of research comprising many unanswered fundamental questions. Here, we examine the crystallization of SSZ-13, which is an aluminosilicate zeolite that possesses exceptional physicochemical properties for applications in separations and catalysis (e.g., methanol upgrading to chemicals and the environmental remediation of NO(x)). We show that SSZ-13 grows by two concerted mechanisms: nonclassical growth involving the attachment of amorphous aluminosilicate particles to crystal surfaces and classical layer-by-layer growth via the incorporation of molecules to advancing steps on the crystal surface. A facile, commercially viable method of tailoring SSZ-13 crystal size and morphology is introduced wherein growth modifiers are used to mediate precursor aggregation and attachment to crystal surfaces. We demonstrate that small quantities of polymers can be used to tune crystal size over 3 orders of magnitude (0.1-20 μm), alter crystal shape, and introduce mesoporosity. Given the ubiquitous presence of amorphous precursors in a wide variety of microporous crystals, insight of the SSZ-13 growth mechanism may prove to be broadly applicable to other materials. Moreover, the ability to selectively tailor the physical properties of SSZ-13 crystals through molecular design offers new routes to optimize their performance in a wide range of commercial applications.

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

Zeolites containing Sn, Ti, Zr, Hf, Nb, or Ta heteroatoms are versatile catalysts for the activation and conversion of oxygenated molecules owing to the unique Lewis acid character of their tetrahedral metal sites. Through fluoride-mediated synthesis, hydrophobic Lewis acid zeolites can behave as water-tolerant catalysts, which has resulted in a recent surge of experimental and computational studies in the field of biomass conversion. However, many open questions still surround these materials, especially relating to the nature of their active sites. This lack of fundamental understanding is exemplified by the many dissonant results that have been described in recent literature reports. In this review, we use a molecular-based approach to provide insight into the relationship between the structure of the metal center and its reactivity toward different substrates, with the ultimate goal of providing a robust framework to understand the properties that have the strongest influence on catalytic performance for the conversion of oxygenates.