Catalytic arene alkylation over H-Beta zeolite: Influence of zeolite shape selectivity and reactant nucleophilicity

Abstract

Renewable arenes and aromatic alcohols can be derived from lignocellulose by biorefineries, which has been considered as a sustainable alternative to replace petrochemical feedstocks in the synthesis of monobenzylation products, key industrial intermediates, via benzylation reactions. Zeolites with micropores are the most widely used catalysts in the benzylation of arenes, however, their performance suffers from diffusion limitations in converting large arenes. In this work, mesoporous and microporous H-Beta zeolites were prepared and applied in the systematic study of benzylation of arenes (benzene, toluene, p-xylene and mesitylene) with benzyl alcohol (BA). The porous structure of these zeolites has been confirmed by XRD, BET and TEM techniques. The catalytically active Brønsted acid sites (BAS) were determined by quantitative 1 H magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments. The benzylation studies have shown that introducing mesopores into H-Beta zeolites can significantly increase the diffusion/access of arenes to surface sites, particularly for bulky arenes (e.g. mesitylene), while micropores are mainly selective for the conversion of small arenes (e.g. benzene).Increasing the nucleophilicity of arenes with more alkyl groups can enhance their catalytic performance in mesopores, however, the increase hinders their conversion in micropores because of the shape selectivity due to their increasing molecular size. Compared to mesopores, micropores promote the conversion of small arenes (e.g. benzene), which can be additionally enhanced by a high Brønsted acidity. Therefore, introducing a suitable porosity balanced with acidity is are keys in the tailoring of the catalytic performance of H zeolites for target benzylation reactions.

Qualitative and Quantitative Analysis of Acid Properties for Solid Acids by Solid-State Nuclear Magnetic Resonance Spectroscopy

“…The increase in Δδ 1 H is in line with the acid strength of bridging OH groups in zeolites, such as 5.1, 7.1, and 7.9 ppm for zeolites HY, H-Beta, and H-ZSM-5. 6,23,121,175 On the contrary, Δδ 1 H increases at a higher framework Si/Al ratio of zeolite H-Beta, 6,23 corresponding to a higher acid strength attributed to the increase in the electronegativity of the framework. 220,221 Acetone-2-13 C is one of the probe molecules widely used for evaluating the strength of both BAS and LAS.…”

“…11−14 Compared with solely BAS or LAS, the cooperativity of BAS and LAS can promote catalytic reactions via enhancing the acid strength, tuning and integrating multiple acid-catalyzed reaction steps. 15−22 In the reactions of large molecules, the catalytic performance of solid acids strongly depends on the accessibility of acid sites to reactant(s); 23,24 for instance, a ∼20 times higher turnover frequency (TOF) was obtained with surface acid sites on amorphous silica−aluminas compared with that on microporous dealuminated HY zeolite in the conversion of phenylglyoxal. As excellent supports, the density and strength of acids sites on solid acids are key to stabilizing and tuning the catalytic properties or cooperating with metal active sites for selective hydrogenation and oxidation reactions.…”

Niobium on BEA Dealuminated Zeolite for High Selectivity Dehydration Reactions of Ethanol and Xylose into Diethyl Ether and Furfural

“…With the increase in the percentage of Nb over DB, there was an increase in the percentage of Al Oh. In catalytic reactions, Al Td can be characterized as a strong Brønsted acid site and Al Oh as a Lewis acid site (located outside the framework) [ 4 , 34 ]. Thus, the addition of niobium on DB zeolite promoted a decrease of Al atoms from the framework during thermal treatments, as observed by supported Nb 2 O 5 /ZSM-5 [ 35 , 36 ].…”

“…After the modifications made to the catalysts, there was no significant shift in the Q 3 signals around −102 ppm, which refers to Si (1Al, 3Si or SiOH) in different environments and neither in Q 4 (around −111 and −115 ppm, which can be associated with the Si (0Al) environment. Zeolite *BEA presented nine different crystallographic sites [ 37 ], which can cause different chemical shifts to the same Si (nAl) environment [ 28 , 34 , 38 ]. The major modification of Q n distribution was from HB to DB, with the decreased amount of Q 3 .…”

Pentacoordinated Aluminum Species: New Frontier for Tailoring Acidity-Enhanced Silica–Alumina Catalysts

“…However, biomass reactions often involve reactions of large molecules, resulting in mass transfer issues inside the small pores and channels of zeolites. 39 Nonporous ASAs provide virtually unconstrained diffusion of reactants/products to and from surface sites and are thus of great interest for biomass conversions. For example, dealuminated HY (De-Al-HY) zeolites are considered as the most active solid acids in the production of alkyl mandelates via redox disproportionative conversion of aromatic aldehydes such as phenylglyoxal (PG), 40 which are important chiral building blocks in the synthesis of pharmaceuticals and fine chemicals.…”