Abstract
A systematic silylation approach using mono-, di-, and trichlorosilanes with different alkyl chain lengths was employed to enhance the hydrothermal stability of zeolite Y. DRIFT spectra of the silylated zeolites indicate that the attachment of the silanes takes place at surface silanol groups. Regarding hydrothermal stability under aqueous-phase processing (APP) conditions, i.e., pH ≈ 2, 473 K and autogenous pressure, the selective silylation of the zeolite surface using monochlorosilanes has no considerable influence. By using trichlorosilanes, the hydrothermal stability of zeolite Y can be improved significantly as proven by a stability test in an aqueous solution of 0.2 M levulinic acid (LA) and 0.6 M formic acid (FA) at 473 K. However, the silylation with trichlorosilanes results in a significant loss of total specific pore volume and total specific surface area, e.g., 0.35 cm3 g−1 and 507 m2 g−1 for the silylated zeolite Y functionalized with n-octadecyltrichlorosilane compared to 0.51 cm3 g−1 and 788 m2 g−1 for the parent zeolite Y. The hydrogenation of LA to γ-valerolactone (GVL) was conducted over 3 wt.-% Pt on zeolite Y (3PtY) silylated with either n-octadecyltrichlorosilane or methyltrichlorosilane using different reducing agents, e.g., FA or H2. While in the stability test an enhanced hydrothermal stability was found for zeolite Y silylated with n-octadecyltrichlorosilane, its stability in the hydrogenation of LA was far less pronounced. Only by applying an excess amount of methyltrichlorosilane, i.e., 10 mmol per 1 g of zeolite Y, presumably resulting in a high degree of polymerization among the silanes, a recognizable improvement of the stability of the 3 PtY catalyst could be achieved. Nonetheless, the pore blockage found for zeolite Y silylated with an excess amount of methyltrichlorosilane was reflected in a drastically lower GVL yield at 493 K using FA as reducing agent, i.e., 12 vs. 34% for 3PtY after 24 h.
Highlights
Today’s global economies heavily rely on the utilization of fossil resources
In the O–H stretching vibration region (Figure 1A), the parent zeolite Y displayed a dominant band at 3,738 cm−1, which can be assigned to the free silanol groups, and two other bands characteristic of Brønsted acid sites (BAS) located in the supercages (3,627 cm−1) and the sodalite cages (3,562 cm−1) of the zeolite framework (Weitkamp, 2000)
We demonstrated that selective silylation of the free silanol groups on the external surface is evident by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for zeolite Y silylated with monochlorosilanes
Summary
Today’s global economies heavily rely on the utilization of fossil resources. The most predominant side effect of this dependence is the emission of greenhouse gases resulting in global warming and climate changes. The uncertain supply of crude oil drives the accelerated search for sustainable substitutes in order to reduce the strong reliance on fossil resources In this regard, biomass has been extensively studied as a renewable resource for the production of chemicals and fuels due to its ubiquity (Alonso et al, 2010). Aqueous-phase processing (APP) is a selective and comparatively mild approach to utilize lignocellulosic biomass via hydrolysis and subsequent heterogeneously catalyzed upgrading. In this respect, the highly oxygenated macromolecules as present in lignocellulosic biomass are depolymerized and converted to a number of platform chemicals, including hydroxymethylfurfural (HMF), levulinic acid (LA), 2-methyltetrahydrofuran (Alonso et al, 2013), levulinate esters (Sun and Cheng, 2002), δaminolevulinic acid (Chheda et al, 2007), or γ-valerolactone (GVL) (Cortright et al, 2002; Huber et al, 2004; Gallezot, 2012). For Pt catalysts on selected silylated zeolites the activity and selectivity toward GVL were assessed in the aqueousphase in-situ hydrogenation of LA using different reducing agents, i.e., FA or H2
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