Formaldehyde is one of the most frequently detected VOCs that cause mild-to-chronic effects to humans. It can irritate the skin, eyes, nose, and throat. Moreover, higher levels of formaldehyde have been connected to an increased risk of some cancer types [1,2]. To prevent these effects, advanced technologies such as capturing and converting formaldehyde into useful products have been proposed. However, formaldehyde has become limited in its application for organic synthesis because of its low boiling point (19.5 °C) and it can rapidly polymerize to solid paraformaldehyde and trioxane. Recently, Na-X and Na–Y zeolites were found to be used to preserve formaldehyde in monomeric form without self-polymerization and its reactivity with various nucleophiles via the carbonyl-ene reaction remains unchanged [3]. Considering the trimerization equilibrium, QM calculations reveal that the polymerization of formaldehyde in the gas phase is thermodynamically favorable. In Na-FAU zeolite, the adsorption of monomer on the Na-FAU becomes more thermodynamically favorable than the adsorption of 1,3,5-trioxane on the Na-FAU zeolite. Due to the shift of the equilibrium towards the reactant side, formaldehyde molecules in Na-FAU tend to favor the molecular form [4].

The carbonyl-ene reaction is a well-known organic reaction that involves the addition of a nucleophile, typically an alkene, to the carbonyl group of a carbonyl compound. In the case of formaldehyde (HCHO), the carbonyl-ene reaction can occur with various alkenes, including propylene (C3H6). The reaction proceeds through the attack of the alkene's π bond on the electrophilic carbon of the carbonyl group, leading to the formation of a new carbon-carbon bond and a β-hydroxy aldehyde product [[5], [6], [7]]. The carbonyl-ene reaction between formaldehyde and propylene represents a fundamental and well-known example of this versatile transformation which is generally induced by Lewis acids [8,9]. 3-Buten-1-ol is the simplest product of the carbonyl-ene reaction. It is used as a monomer in polymerization reactions and as an intermediate in the synthesis of tetrahydrofuran (THF).

Porous materials like metal-organic frameworks (MOFs) become more advantageous because they exhibit the Lewis acid property together with the accessible variation of pore sizes and are environmentally friendly. These materials contain metal ions or metal oxide clusters coordinated to organic linkers to form ordered frameworks [[10], [11], [12], [13], [14]]. HKUST-1 or Cu–BTC is one of a subclass MOFs that contain coordinately unsaturated active sites (CUS) [15]. It consists of Cu paddlewheel nodes linked by carboxylate groups (Fig. 1). Such metal unsaturated sites can perform as Lewis acid sites that react toward adsorbing molecules. These materials show a high potential for gas adsorption and separation [16,17]. They are active catalysts for various Lewis-acid-catalyzed reactions [[18], [19], [20], [21], [22]]. Maihom et al. investigated the effect of five M3(BTC)2 (M = Fe, Co, Ni, Cu, and Zn) MOFs catalysts on the catalytic activity of the carbonyl-ene reaction between encapsulated formaldehyde and propylene using DFT calculations. They found the relative catalytic activity based on activation energy and turnover frequencies (TOF) to be Zn3(btc)2 > Fe3(btc)2 > Co3(btc)2 > Ni3(btc)2 > Cu3(btc)2 [23].

Not only changing the type of metal cation in paddlewheel units, but the doping of secondary metal ions into paddlewheel units to form bimetallic paddlewheel-based MOFs is also an alternative way to enhance the catalytic properties of parent HKUST-1 MOFs. Many studies have investigated the effect of the substitution of secondary metal ions in the HKUST-1 framework on catalytic reactions. The mixed-node A-Cu-BTC (A = Sr, La, Ce, and Al) were successfully synthesized by hydrothermal method and were applied as a catalyst in selective catalytic reduction of NO with CO [24]. The substitution of Mn-, Fe-, and Co-metal centers into Cu-BTC via a post-synthetic process showed better selectivity of O2 and N2 adsorption [25]. The synthesized Mg–Cu-BTC demonstrates the enhancement of CO2 adsorption as compared to the parent Cu-BTC significantly [26]. Furthermore, it was found that the doping of alkaline earth metal ions (Mg2+ and Ca2+) can enhance the water uptake capacity as compared to the parent HKUST-1 [27]. The substitution of copper ions with alkaline earth metal creates more exposed active sites for water sorption due mainly to the increased radius of ions. Recently, Sirijaraensre studied the structures, electronic properties, and catalytic activity of bimetallic M-Cu-BTC paddlewheels (M = Mg, Ca, Al, and Ga) for styrene oxide (SO) cycloaddition with CO2 and found that the substituted metal centers act as key active sites for this reaction and stronger interaction of SO molecule on the substituted metal center is discovered as compared to the parent Cu-BTC [28]. To the best of our knowledge, the catalytic performance of mixed-metal M-Cu-BTC substituted by group IIA (Be, Mg, and Ca) for formaldehyde encapsulation and catalyzing the carbonyl-ene reaction has not been investigated either experimentally or theoretically.

In this work, the carbonyl-ene reaction between formaldehyde and propylene molecules is investigated on partially substituted paddlewheel Cu-BTC with group IIA metals (Be, Mg, and Ca) using the M06-L density functional. The electronic structure of these mixed-metal paddlewheels was studied. We investigated the effect of substituted metals on the encapsulation performance of modified Cu-BTC. Additionally, we discussed the reaction mechanism and energetic profile, and their reaction complex structures compared to the related systems. The calculated results could be a guild to predict the promising catalytic materials to encapsulate formaldehyde and convert it to a more value-added compound.