With Open Arms: Open Sites of ZrBEA Zeolite Facilitate Selective Synthesis of Butadiene from Ethanol

Abstract

Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations have been used to elucidate the nature of active sites of ZrBEA zeolite responsible for the catalytic synthesis of butadiene. We show that the content of open Zr(IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three -O-Si linkages and one OH-group, correlates with the catalytic activity in the process of conversion of ethanol into butadiene. Higher catalytic activity of the open sites is attributed to their higher acid strength and steric accessibility. The study suggests that the control of such open sites plays a crucial role for the further design of the optimal multifunctional zeolite-based catalysts.

Recent Breakthroughs in the Conversion of Ethanol to Butadiene

“…On the contrary, the relative amount of closed sites did not correlated with the conversion of ethanol, suggesting that they are either inactive or less active in the ETB reaction [49]. [32,49]. A direct correlation between the relative number of open Lewis acid sites and the initial rate of butadiene production was observed ( Figure 10).…”

“…When available, this paper will also indicate the TOS at which the activity was recorded. It has been repeatedly demonstrated that the main culprit behind catalytic deactivation was the formation of coke on the catalytic surface [4,36,47,49,50]. Table 1 highlights the performances of notable catalysts for the one-step process reported in the past hundred years to provide references to the readers (catalysts 1 to 10 in Table 1), as well as the catalytic systems reviewed in this work.…”

“…Although Ag/Zr/BEA and Ag/Zr/MCM-41 displayed similar activity, the latter generated more dehydration products due to the greater amount of surface silanol groups acting as Brønsted acid sites. The nature of Zr/BEA active sites was further investigated in subsequent studies [49,70]. Ivanova et al argued that the configuration of Zr(IV) within the zeolite framework influenced the activity of the Lewis acid sites-a phenomenon comparable to the well-documented cases of Ti/BEA and Sn/BEA in which distinguishable "open" and "closed" Lewis acid sites have been demonstrated to exist [71][72][73][74][75].…”

“…It was therefore concluded that both signals belong to the same species: the open Lewis acid sites, and that CDCl 3 exclusively adsorbed unto it, sterically hindering the access to CO. Only the latter could properly distinguish between both sites. With a method to detect Zr open and closed sites, Ivanova et al were able to compare their relative amounts with the formation of butadiene from ethanol obtained during catalytic testing [32,49]. A direct correlation between the relative number of open Lewis acid sites and the initial rate of butadiene production was observed (Figure 10).…”

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

“…Other heteroatoms, such as Zr, Hf, and Ti, show only one band in the 2,305-2,315 cm −1 range. Sushkevich et al (69,100) recently reported that acetonitrile preferentially binds to ZrBeta open sites and showed that CO adsorption at low temperature (∼100 K) could be used to differentiate between open and closed sites in Zr-Beta (see Figure 4d). The bands at 2,185 cm −1 and 2,176 cm −1 are assigned to CO adsorbed onto the open and closed sites, respectively.…”