Zeolites are widely used in various acid-catalyzed reactions (e.g., cracking, disproportionation, isomerization, and alkylation) in the chemical and petrochemical industry due to their peculiar pore structure, strong acidity, and high selectivity. [1][2][3][4] Since the catalytic activity and selectivity of dealuminated zeolites are much higher than those of their respective parents, zeolite modification by dealumination has received considerable attention. [5][6][7] In zeolites, fourcoordinate framework aluminum (FAL) is associated with a Brønsted acid site (SiOHAl), while extra-framework aluminum (EFAL) species generated during the dealumination process acts as a Lewis acid site. The existence of EFAL species is crucial for a favorable influence on the catalytic properties of zeolites.[8] Although enormous progress has been made in the studies of the nature of both FAL and EFAL by various methods, including solid-state NMR spectroscopy, [9] X-ray standing waves, [10] X-ray absorption near edge structure, [11] and theoretical calculations, [12] the detailed structure of EFAL species and the spatial proximities (or interactions) of various Al species in dealuminated zeolites are poorly understood. This strongly hampers the understanding of structure-activity relationship in numerous zeolites.One-dimensional single-pulse 27 Al MAS NMR and twodimensional multiple-quantum magic angle spinning (MQ-MAS) NMR have been used extensively to study the local symmetry and coordination state of aluminum species in zeolites. [13][14][15] However, both are unable to obtain information on the spatial correlation of different aluminum species. Twodimensional 27 Al double-quantum MAS NMR (DQ-MAS NMR) is a powerful technique for probing aluminumaluminum proximities in solid materials. However, the 27 Al DQ-MAS NMR technique still remains a great challenge because of the quadrupolar nature of the aluminum nucleus (I = 5/2), which leads to low efficiency of DQ excitation (< 5 %). So far, the 27 Al DQ-MAS NMR technique has been successfully applied to systems with high Al content, such as aluminophosphate molecular sieves, [16][17][18] glasses, [19] and minerals, [20] but for aluminosilicate zeolites with low Al content, the technique was less successful due to its extremely low sensitivity. [21] Homonuclear dipolar recoupling of quadrupolar nuclei under MAS is difficult because of the intricate nuclear spin dynamics of the quadrupolar nuclei in the presence of an rf field and sample rotation. Mali et al. first demonstrated that the rotary resonance recoupling (R 3 ) technique with HORROR condition [22] can be used for DQ recoupling of half-integer quadrupolar nuclei. [16,23] As an improvement, EdØn et al. then showed that symmetry-based pulse sequences display superior rf error tolerance than HORROR recoupling, and these sequences were incorporated into DQ-MAS experiments. [17,20] Recently, we developed a new homonuclear 27 Al DQ-MAS NMR correlation method based on the rotor-synchronized and symmetry-based BR2
