Abstract
The generation of Brønsted (Sn–OH) and Lewis (coordinatively unsaturated metal centers) acidic sites on the solid surface is a prime demand for catalytic applications. Mesoporous materials are widely employed as catalysts and supports owing to their different nature of acidic sites. Nevertheless, the procedure adopted to generate acid functionalities in these materials involves tedious steps. Herein, we report the tunable acidic sites containing Brønsted sites with relatively varied acid strength in tin oxide by employing soft template followed by simple thermal treatment at various temperatures. The readily accessible active sites, specifically Brønsted acidic sites distributed throughout the tin oxide framework as well as mesoporosity endow them to perform with exceptionally high efficiency for epoxide ring opening reactions with excellent reusability. These features promoted them to surpass stannosilicate catalysts for the epoxide ring opening reactions with alcohol as a nucleophile and the study was extended to aminolysis of epoxide with the amine. The existence of relatively greater acid strength and numbers in T-SnO2-350 catalyst boosts to produce a high amount of desired products over other tin oxide catalysts. The active sites responsible in mesoporous tin oxide for epoxide alcoholysis were studied by poisoning the Brønsted acidic sites in the catalyst using 2,6-lutidine as a probe molecule.
Highlights
The generation of Brønsted (Sn–OH) and Lewis acidic sites on the solid surface is a prime demand for catalytic applications
The total number and Brønsted acid sites in T-SnO2-x decreases with increase in the calcination temperatures >350 °C attributed to the dehydroxylation of surface Sn–OH moieties which results in –Sn–O–Sn– yielding SnO2 containing lower –OH moiety which acts as Brønsted acidic sites
We performed a comprehensive assessment on easier synthesis of porous tin oxide and expandable variants in them which offers tailored properties different from conventional mesoporous catalysts in many characteristics, such as tunable structure and chemical properties by simple thermal treatment
Summary
The generation of Brønsted (Sn–OH) and Lewis (coordinatively unsaturated metal centers) acidic sites on the solid surface is a prime demand for catalytic applications. The number of acidic sites in tin oxide was confirmed from NH3-TPD (Table 1, ESI Fig. S5) and the measurements were performed till its calcination temperature as catalyst may give peaks unrelated to NH3 desorption at higher temperatures causing erroneous result. The total number and Brønsted acid sites in T-SnO2-x decreases with increase in the calcination temperatures >350 °C attributed to the dehydroxylation of surface Sn–OH moieties which results in –Sn–O–Sn– yielding SnO2 containing lower –OH moiety which acts as Brønsted acidic sites.
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