Catalyst deactivation caused by coking has always been a concern in the aldol condensation reaction. In this work, Ba–La-modified alumina has been characterized and tested as a catalyst in the vapor-phase aldol condensation process. The coke formation, acid–base properties, and deactivation behavior of the spent catalysts were studied through a series of characterization techniques: N2 adsorption–desorption, pyridine FT-IR, TG-DTA, TPO-MS, as well as NH3, CO2-TPD, and UV Raman spectroscopy. It was found that the main components of coke are polycyclic aromatic hydrocarbons, which can be removed by high-temperature roasting under an air atmosphere. The overactivation of formaldehyde accelerates the rate of coke deposition and ultimately aggravates the decrease in the stability of the catalyst. Furthermore, adding a proper amount of methanol to the reactants can greatly inhibit the hydrolysis of esters and improve the selectivity of the catalyst for the target reaction. Under the optimum reaction condition of Ma/FA/CH3OH = 6:2:8, the pairs of medium acidic and weakly basic sites enable Ba–La/Al2O3 to achieve the highest yield of methyl acrylate.

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In this paper, a series of alkaline-treated ZSM-22 zeolite samples were prepared by treating the parent ZSM-22 zeolite using NaOH aqueous solution with different concentrations. By investigating the effects of alkaline treatment on the parent ZSM-22 zeolite, we discovered that the alkaline treatment contributed to the reduction of Brønsted acid sites due to the coverage of extra-framework Al on its external surface. In addition, it was found that the alkaline-treated samples were favorable to the improvement of the isobutene yield and selectivity, while these features appeared to be low for the subsequent acid-washed counterparts in the skeletal isomerization reaction of 1-butene. These results indicate that the catalytic performance of ZSM-22 zeolite is related to reduced amounts of Brønsted acid sites in it. To further reveal the reasons for the promoted catalytic performances of the alkaline-treated ZSM-22 series zeolites, we studied the properties of coke deposited on the two series of samples using Raman spectroscopy and thermogravimetric analysis and mass spectrometry (TG/MS-TPO). It was shown that the carbon deposited on the alkaline-treated series samples was mainly distributed at the outer surface, while the coke was distributed to a relatively lesser extent at the exterior surface for the acid-washed series samples. Moreover, by partially passivating outer acid sites of the parent zeolite, the selected alkaline-treated zeolite, and acid-washed zeolite, their isobutene selectivities were all improved with the decrease in outer acid sites. These phenomena confirmed that the improved catalytic performances of the alkaline-treated samples are related to their decreased external Brønsted acid site density, which further demonstrated that the high isobutene yield and selectivity in the skeletal isomerization reaction of 1-butene is realized via the monomolecular reaction pathway of 1-butene.