Methylation of phenol over high-silica beta zeolite: Effect of zeolite acidity and crystal size on catalyst behaviour

Abstract

A systematic investigation has been carried out, aiming at elucidating several aspects of the gas/solid methylation of phenol over high Si/Al ratio BEA-structured zeolite in protonated form. The catalysts have been characterized by several techniques, such as XRD, SEM, BET, ICP, FT-IR, TGA, micro-calorimetry and modelling by ab initio calculations. The correlation between these characteristics and kinetics and mechanistic features of the catalytic reaction, as well as of catalyst deactivation, showed that these zeolites are very active for the present reaction, leading to cresols and anisole as primary products. As catalyst deactivation proceeds, the selectivity to cresols and anisole increases substantially, together with a rapid decrease of selectivity to poly-alkylated species. Catalyst surface acidity is prevalently made of medium-to-low-strength silanolsbased acid sites of Brønsted type. High-strength Lewis acid sites are either almost absent, especially when metal cations partially substitute for protons, or play a role essentially in catalyst deactivation. Stacking faults in the zeolite framework, generated by the 2 intergrowth of at least two BEA polymorphs, lead to an increase of the concentration of silanols-based acid sites. Deactivation is essentially due to the interaction of phenol and oxygenated products with the strong Lewis acid sites. For time-on stream values longer than a few hours, self oligomerisation-cyclisation of methanol to olefins and aromatics, followed by further alkylation to aromatic C atoms, contributes more significantly to catalyst deactivation. At higher temperature all the zeolites deactivate at a comparable rate, whereas at lower temperature initial catalytic activity is higher for larger crystal size zeolite, due to the longer diffusion time of bulkier coke precursors within zeolite pores. At any conversion level and at any temperature the anisole/cresols ratio is systematically lower for the larger crystal size zeolite, since the secondary transformations of anisole to cresols, by both intra-molecular rearrangement and inter-molecular alkylation of phenol, is favoured by the longer residence time of anisole within the zeolite pores.

Study of the Alkylation of Phenol with Methanol on Zn(H)-Exchanged NaY Zeolites

“…Formation of heavy dialkylated products, in particular xylenols, may also form carbonaceous products and contribute to catalyst deactivation. Indeed, the retention of polyalkylated phenols has been proposed as one of the mechanism responsible for the progressive deactivation of the catalyst in the methylation of phenol [13]. The observed correlation in Table 3 between X 0 P and d 0 suggests in fact that the catalyst activity decay increases with the formation of heavier secondary products at high conversion levels.…”

“…Solid bases, in particular MgO, selectively alkylate phenol in ortho-position, producing essentially o-cresol and 2,6 xylenol [5,6]. Solid acids such as silicaalumina, Nafion-H resin, HPA/SiO 2 , and zeolites HBEA and HY, convert phenol to a mixture of anisole, cresols, xylenols and methylanisoles [7][8][9][10][11][12][13]. Brønsted solid acids promote the O-alkylation of phenol [10,11] while Lewis solid acids (SiO 2 -Al 2 O 3 ) and zeolites containing similar amounts of Lewis and Brønsted acid sites (HBEA, HY) favor the C-alkylation of phenol [7,14].…”

The transformations involving methanol in the acid- and base-catalyzed gas-phase methylation of phenol

“…In a recent work [61] we reported that in the gas-phase methylation of phenol catalyzed by H-BEA zeolites, the main products of methanol transformation were pentamethyl and hexamethylbenzene; the latter, and polynuclear aromatics as well, were responsible for the progressive catalyst deactivation.…”

“…A further increase of temperature led to a decrease of conversion. This can be attributed to two concomitant phenomena, (i) a progressive deactivation of the catalyst, due to the formation of condensed aromatics (coke precursors) by transformation of methanol [61][62][63][64][65][66], and (ii) a considerable increase in the parallel reactions of methanol decomposition to light compounds. Indeed, the spent catalyst was coked, and its surface area was considerably lower than that one of the fresh catalyst ( Table 1).…”

“…In the case of the H-BEA zeolite, the main products in methanol transformation at high temperature were light aliphatics, both saturated and unsaturated, hexamethylbenzene and pentamethylbenzene [61], while ZSM-5 gave mostly dimethyl and trimethylbenzenes [62]. These compounds were further converted to naphthalene derivatives that were finally responsible for the formation of coke precursors and the deactivation of the zeolite [63].…”