In-situ regulating silanol defects and acid properties of ZSM-48 zeolite by inorganic acid for efficient selective hydroisomerization of long-chain alkanes

In-situ acid regulation tailors silanol defects and acidity of ZSM-48. • Gel pH and Al coordination control defects and Brønsted acidity. • Framework coordination is tuned without altering *MRE topology. • Pt/ZSM-48-N achieves 87.3% isomer yield in n-hexadecane hydroisomerization. • SISSO and DRIFTS reveal that defect-acidity synergy suppresses cracking. ZSM-48 zeolite, featuring a one-dimensional ten-membered-ring channel system, exhibits excellent support properties for hydroisomerization of long-chain alkanes. However, the intrinsically unbalanced distribution of acid sites significantly increases the probability of undesired cracking side reactions, making it challenging to simultaneously achieve high isomer selectivity at high conversion level. In this work, an in-situ acid-regulated crystallization strategy is developed to markedly enhance the hydroisomerization performance of ZSM-48. A series of ZSM-48-X samples are synthesized under different acid-regulated conditions, of which the physicochemical properties, particularly silanol nest concentration and acid characteristics, exhibit a strong dependence on the initial synthesis pH and type of anion in acid. The resulting ZSM-48-X zeolites with increased silanol nest density and reduced strong Brønsted acid sites exhibit substantially improved catalytic performance, achieving a general increase in isomer yield from 77.7% to above 85%. Symbolic regression of catalyst characteristics and reaction data by sparse identification of selected operators (SISSO) demonstrates the primary roles of silanol nests and strong Brønsted acid sites on facilitating isomerization reaction in a nonlinear manner. In-situ DRIFTS experiments on the catalysts further demonstrate that abundant silanol nests and optimized density of Brønsted acid sites in ZSM-48 weaken the adsorption strength of long-chain alkanes and promote the desorption of isomerized products, providing molecular-level mechanism insights into the promotional function of acid-regulated ZSM-48 for hydroisomerization determined by SISSO. This in-situ acid-regulated crystallization strategy broadens the scope of conventional zeolite synthesis and acidity modulation, offering a simple and efficient approach for tailoring silanol nests and providing valuable guidance for rational design of high-performance zeolite catalysts. Download: Download high-res image (200KB) Download: Download full-size image