Research Article pubs.acs.org/acscatalysis
Water Interactions in Zeolite Catalysts and Their Hydrophobically Modified Analogues Kuizhi Chen, Jarred Kelsey, and Jeffery L. White* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, United States
Lu Zhang and Daniel Resasco Department of Chemical, Materials, and Biological Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States S Supporting Information *
ABSTRACT: Renewed interest in zeolite catalyst performance in the presence of variable amounts of water has prompted solid-state NMR experiments designed to identify the nature of water interaction with and within conventional and chemically modified H-ZSM-5 zeolites. Recent work has demonstrated that water can positively influence reaction rates in zeolite-catalyzed chemistries, and new interest in catalytic processing of molecules derived from biomass requires understanding the fate of water in and on zeolite catalysts, as a function of water loading. The contribution of acid site density to water adsorption within zeolites is assessed by comparing bulk uptake and molecular experiments at varying Si:Al ratios, and interpreting those results in the context of solid-state NMR results that reveal strongly adsorbed water molecules and water clusters. In situ magic-angle spinning (MAS) NMR experiments for water loadings ranging from ca. 4 to 500 water molecules per zeolite unit cell indicate the following: (1) the dominant interaction is from water adsorbed from the vapor phase at an interior acid site, and unique signals for both the water and acid site are resolved at low loadings; (2) the exchanged-averaged water/acid site chemical shift at higher loadings can be used to measure acid site titration by water; and (3) silane-treated hydrophobically modified H-ZSM-5 does not allow liquid-phase water to access interior acid sites. The in situ 1 H MAS NMR method indicates that as-synthesized acidic zeolites can be rendered hydrophobic in the presence of liquid-phase water, with only a minimal reduction in the total number of acid sites. KEYWORDS: zeolite, catalysis, water, in situ, solid-state NMR
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INTRODUCTION A resurgent interest in the behavior of water in zeolite catalysts reflects a new understanding that water can positively influence reaction rates and product selectivity in some heterogeneous catalysis systems,1−5 and also from the desire to convert molecules derived from biomass using traditional or modified zeolite catalysts.6−10 Water can potentially enhance the proton transfer rate via a bimolecular reaction pathway involving lower energy transition states, as has been recently discussed for a metal-catalyzed reactions in Fischer−Tropsch catalysis.1 Increased hydrocarbon C−H bond activation rates have recently been reported in the presence of low amounts of water.3 Biomass conversion generates molecules that are highly oxygenated, and thus may evolve significant amounts of water in subsequent reactions. Zeolites are attractive routes for their catalytic upgrading, but traditional vapor-phase chemistries pose problems for cellulosic and saccharide-based feedstocks. Recently, Resasco and co-workers have shown that zeolite catalysis in water is a viable route for conversion of some biomass molecules, based on hydrophobic modifications of zeolite crystallites designed to prevent their dealumination in an aqueous solvent environment.11,12 Other recent reports suggest © XXXX American Chemical Society
that water can also play a role in realumination of framework sites in zeolites.13 The degree to which water positively or negatively influences reaction chemistries should be dependent on the relative water loading present near the solid acid site in a zeolite under reaction conditions. In addition, the dynamics of water diffusion will also contribute to the impact of co-adsorbed water on reactions, i.e., water which is simultaneously proximate to the acid site in the presence of reagent and product molecules. It has been suggested that water provides an assisting role in stabilizing transitions states and side-chain elimination reactions in methanol-to-hydrocarbon chemistry, where the origin of this water comes from acid-catalyzed methanol conversion to dimethyl ether.14 This stoichiometric water represents one extreme in reaction conditions, in which water is produced from the reagent at the active site, which can be contrasted to the case where water is present as a solvent or as part of a biphasic solvent system. Thus, a fundamental molecular Received: September 14, 2015 Revised: November 4, 2015
7480
DOI: 10.1021/acscatal.5b02040 ACS Catal. 2015, 5, 7480−7487
Research Article
ACS Catalysis
Figure 1. Gravimetrically determined water uptake rates and maximum loadings for dehydrated acidic HZSM-5 catalysts exposed to ambient moisture, plotted as a function of (a) number of water molecules per unit cell, and (b) number of water molecules per acid site. Different Si/Al ratios are identified in each legend. The catalyst particle bed thickness was 1−2 mm (on average).
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understanding of how water interacts with, and diffuses within, hydrophilic solid acid zeolite catalysts versus their hydrophobically modified analogues is directly relevant to the timely questions raised above. In this contribution, solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques are used to examine the behavior of water in acidic HZSM-5 zeolites with different Si/Al as a function of water loading, ranging from ∼1 water molecule per acid site up to ca. 500 water molecules per unit cell. The adaptation of MAS NMR methods to study water adsorption and interaction in zeolites has been discussed previously in the literature, albeit in the absence of comparisons to hydrophobically modified catalytic analogues.15−17 In this contribution, experiments were designed to (1) directly probe the interaction of water with the interior surface of acidic zeolites, including the acid site itself, (2) measure the interactions as a function of water loading and acid site density, and (3) investigate whether specific chemical modifications designed to increase catalyst stability in water alters the molecular-level behavior of water within the catalyst particles. We show that molecular-level in situ experiments separately resolve many species, including free acid sites (which are the locus of catalytic activity), water strongly adsorbed at acid sites and thus inside the catalyst, and extracrystalline water that exists outside of the catalyst. Adsorption of water into the intracrystalline volume of the catalyst, and thus proximate to acid sites, is independent of whether the water originates in the vapor or liquid phase for H-ZSM-5. However, the interaction of water with acid sites in the hydrophobic version of H-ZSM-5, created via reaction of the neat catalyst with either ethyltrichlorosilane (ETS) or octyltrichlorosilane (OTS), only occurs with water in the vapor phase; exposure to liquid water results in water/active site interactions only after vaporization of that liquid water. In either catalyst system, when water is in the vapor phase, the dominant interaction is with the Bronsted acid site, not silanol groups. These results are observed spectroscopically, and cannot be discerned from bulk water uptake measurements. Hydrophobic modification of the H-ZSM-5 with ETS reduces the effective number of acid sites for water interaction by ∼20%, while the larger OTS only reduces it by ca. 10%.
EXPERIMENTAL SECTION
Zeolite Activation. Zeolite ZSM-5 samples with different Si/Al ratios15 (CBV 3024E), 25 (CBV 5524G), 40 (CBV 8014), and 140 (CBV 28014)were obtained from Zeolyst in the ammoniumexchanged form. As reported, the average crystallite size of all these samples is ∼1 μm, the BET surface areas vary from 379 m2/g to 386 m2/g, independent of Si/Al ratios.18 Calcined and dehydrated zeolite samples were prepared from the ammonium form in a glass reactor body using a gradual, stepwise vacuum calcination up to a final temperature of 450 °C, then held at 450 °C for 8 h. High vacuum conditions with pressures of
