Acid site requirements and reaction pathways in the synthesis of hydroxyacetophenones by gas-phase acylation of phenol with acetic acid

C.L. PADRÓ, M.E. SAD AND C.R. APESTEGUÍA

Puerto Vallarta, Mexico

Simposio; 5th International Symposium on Acid Base Catalysis; 2005

Introduction Synthesis of ortho- (o-HAP) and para- (p-HAP) hydroxyacetophenones, which are useful intermediates in the manufacture of important pharmaceuticals, by acylation of phenol with acetic acid is achieved on either liquid or gas phases. In liquid phase, the reaction produces mainly p-HAP using either Friedel-Crafts or solid acid catalysts; but the process is hampered because of environmental constraints and catalyst activity decay.  In gas-phase, the phenol acylation on solid acids forms predominantly o-HAP, but reported experimental o-HAP yields are still moderate, particularly because of significant formation of phenyl acetate.  In this work we study the gas-phase acylation of phenol with acetic acid over different solid acids with the aim of ascertaining the exact requirements of acid site density and strength to efficiently promote the selective o-HAP formation. Experimental The catalysts employed were SiO2-Al2O3, Al-MCM-41, tungstophosphoric acid (HPA) supported on carbon, and zeolites HY, NaY, HBeta, and HZSM5. The gas phase acylation of phenol (P) with acetic acid (AA) was carried out in a fixed-bed reactor at 553 K and 101.3 kPa.  Main reaction products were phenylacetate (PA), o-HAP, and p-HAP.     Results and discussion Surface areas and acidity characterization of the samples are shown in Table 1.  Sample HPA/C showed a NH3 desorption peak at about 910 K which accounts for the strong Brönsted acid sites present on this material.  The evolved NH3 from HY, HZSM5, and SiO2-Al2O3 samples gave rise to a peak at 483-493 K and a broad band between 573 and 773 K. In contrast, HBeta, Al-MCM-41, and NaY samples did not exhibit the high-temperature NH3 band but a single asymmetric broad band with a maximum around 480-496 K. The density and nature of surface acid sites were determined from the FTIR spectra Table 1: Surface areas, acidity, and catalytic results of the catalysts used in this work.  Reaction conditions:  T= 553 K, =146 g h/mol; P/AA=1, N2/(P+AA) = 45 Catalyst Sg m2/g TPD NH3 mmol/m2 FTIR Pyridine Area/g Catalytic results at t = 0 B L X0 S0o-HAP S0p-HAP S0PA HY 660 2.1 310 465 15.0 69.1 8.9 22.0 HZSM5 350 2.2 337 341 18.2 64.1 3.7 32.3 HBeta 560 0.9 150 151 12.7 65.1 4.6 30.3 NaY 700 0.4 n. d. 525 15.9 62.0 0 38.0 Al-MCM-41 925 0.4 32 135 12.9 52.3 1.2 46.5 SiO2-Al2O3 560 1.8 68 204 16.0 39.1 1.6 58.3 HPA/C 390 0.7 - - 15.3 37.0 1.5 61.0 of adsorbed pyridine.  The pyridine absorption spectrum on zeolite NaY did not reveal the presence of surface Brönsted sites.  The amount of pyridine adsorbed on Al-MCM-41 was clearly lower as compared to acid zeolites or SiO2-Al2O3 reflecting its moderate acidic character.  The areal peak relationship between Lewis and Brönsted sites, L/B, was between 1.0 and 1.5 on zeolites HY, HBeta, and HZSM-5, but significantly higher on Al-MCM-41 (L/B = 4.2) and SiO2-Al2O3  (L/B = 3.0).   Figure 1: Simplified reaction network for the synthesis of o-HAP from P and AA on solid acids In a previous paper [1], we identified the primary and secondary reaction pathways involved in the synthesis of o-HAP from P and AA on solid acids by determining the effect of contact time on the product distribution. Specifically, we proposed (Fig. 1) that o-HAP is formed from P and AA via two parallel pathways: i) the direct C-acylation of phenol; ii) the O-acylation of phenol forming the PA intermediate which is consecutively transformed to o-HAP via intramolecular Fries rearrangement or intermolecular phenol/PA C-acylation.  Phenol conversion (X0) and selectivities (Si) values obtained here for all the samples at time zero are given in Table 1. The acylating agent or acylium ion CH3CO+ is formed on our solid acids from AA either on Brönsted or Lewis acid sites. Generated electrophile CH3CO+ may then attack the phenol molecule either by electrophilic substitution of the ortho-hydrogen in the aromatic ring forming o-HAP or, alternatively, by O-acylation of the OH group producing PA.  HPA/C, that contain only strong Brönsted acid sites, promote exclusively the O-acylation of phenol forming phenyl acetate which is then consecutively transformed to o-HAP, probably via an intramolecular Fries rearrangement.  Zeolite NaY that contains only Lewis acid sites produces o-HAP with more than 60 % selectivity for a phenol conversion of about 16 %, similarly to the values found on zeolites HY and HZSM5, which contain similar Lewis acid site concentrations but lower L/B ratios than NaY (Table 1).  These results suggested that C-acylation of phenol to yield o-HAP takes place mainly on surface Lewis acid sites.  On zeolites HZSM5 and HY both o-HAP and PA are primary products, but phenol is directly converted to o-HAP at higher (HZSM5) or similar (HY) rates than to PA.  Zeolites HZSM5 and HY, which contain strong Lewis and Brönsted acid sites, produce then o-HAP at high rates because efficiently catalyze the two main reaction pathways leading from phenol to o-HAP, i.e. the direct C-acylation of phenol and the O-acylation of phenol forming the PA intermediate which is consecutively transformed via intermolecular phenol/PA C-acylation. Solid acids of moderate acid strength such as Al-MCM-41 or SiO2-Al2O3 are less active and selective for o-HAP formation than zeolites HY or HZSM5. References C.L. Padró, C.R. Apesteguía, J. Catal. 226 (2004) 308.