# Zr-Site Lewis Acidity Determines Terpenoid Reduction Selectivity

## TL;DR

This study shows how the acidity of zirconium sites in zeolites affects the selectivity of terpenoid reduction reactions.

## Contribution

The paper introduces a method to correlate Zr-site Lewis acidity with reaction selectivity using FTIR spectroscopy.

## Key findings

- Zr-beta with 'closed' sites favors MPV reduction of citronellal to citronellol.
- Zr-beta with 'open' sites promotes carbonyl-ene cyclization to isopulegol.

## Abstract

Lewis acid zeolites, primarily Al-free Zr and Sn silicates,
catalyze
the chemoselective reduction of ketones and aldehydes to the corresponding
alcohols through hydrogen transfer (Meerwein–Ponndorf–Verley
(MPV) reduction). Sn silicates are more active in the MPV reduction
of ketones, whereas Zr silicates are more active in the MPV reduction
of aldehydes. However, the catalytic activity of these zeolites has
not been accurately ascribed to “open” vs. “closed”
Zr sites even though this correlation is crucial for systems whose
substrate structure allows competing reaction pathways. For example,
MPV reduction of citronellal competes with carbonyl-ene cyclization
to isopulegol and acetalization in the citronellal reaction with 2-propanol.
Therefore, we aimed to correlate thoroughly characterized Lewis acid
sites in Zr-substituted beta and MFI zeolites with their selectivity.
For this purpose, we analyzed Zr-zeolite acidity by fourier transform
infrared spectroscopy (FTIR) spectroscopy of adsorbed deuterated acetonitrile
and acetone because deuterated acetonitrile probes “open”
Zr sites without interacting with “closed” sites, but
acetone identifies both “open” and “closed”
sites. Our results showed that Zr-beta rich in Zr “closed”
sites favored MPV reduction. Conversely, Zr-beta rich in “open”
sites and reference catalysts yielded isopulegol as the main product.
Ion exchange of the Zr-beta “open” sites with Na+ cations deactivated these sites, thereby switching the selectivity
to citronellol. In turn, the silanol groups of the catalyst promoted
acetalization, regardless of substituting the heteroelement (Zr or
Sn). These findings demonstrate that Zr-site Lewis acidity determines
terpenoid reduction selectivity, as the relatively weaker Zr-beta
“closed” sites catalyze citronellal MPV reduction to
citronellol, while the relatively stronger Zr-beta “open”
sites catalyze intramolecular carbonyl-ene cyclization to isopulegol.
Moreover, this correlation between selectivity and Zr-site Lewis acidity
may enable us to design specific catalysts, even for systems with
competing reactions, based on quantitative data acquired using our
experimental paradigm.