A comprehensive review on catalytic O-alkylation of phenol and hydroquinone

ABSTRACT

The alkylation process involves two competitive paths of O- and C-alkylation and achieving better selectivity for desired products is a very challenging problem. The development of new process for synthesis of O-methylated products of phenol and dihydric phenols is a subject of high industrial and academic interest. Alkyl phenyl ethers, especially anisole and 4-methoxyphenol, have captivated significant interest due to their increasing applications in pharmaceutical industries. The main emphasis of the present review is to explore the recent development in two catalytic O-alkylation processes. The first process is O-methylation of phenol into anisole and another is selective mono O-methylation of hydroquinone into 4-methoxyphenol. The present article covers O-alkylation methods with methanol and dimethyl carbonate as alkylating agent over various acidic and basic catalytic systems. The catalyst systems analyzed involves Bronsted and Lewis acidic and basic ionic liquids, conventional acids, metal oxides, solid acid and basic catalysts, hydrotalcites, various zeolites and heteropolyacids. The mechanistic behavior of alkylation reactions in presence of different catalytic system is reviewed critically which is important to design new and/or modified catalyst in order to maximize the yield of desired product. Additionally, an influence of reaction parameters, role of catalyst and their active sites on product distribution is described. The review paper gives useful insight for researchers in the field of catalysis and reaction engineering of alkylation reactions. Understandings of the reaction pathways will help in developing reliable kinetic models necessary for process scale-up to industrial scale reactor system.

KEYWORDS:

• O-alkylation
• phenol
• hydroquinone
• dimethyl carbonate
• methanol

Nomenclature

AIPO: Aluminophosphate

AlPO₄: Aluminium phosphate

B: Bronsted

Ba(HSO₄)₂: Barium hydrogen sulfate

BaSO₄: Barium sulfate

BPO₄: Boron phosphate

CaHPO₄: Calcium hydrogen phosphate

Ca₃(PO₄)₂: Calcium phosphate

Cat-A: Zinc aluminate spinels prepared from aluminium isopropoxide

Cat-B: Zinc aluminate spinels prepared from basic aluminium nitrate

Ce: Cerium

Cs: Cesium

CSTR: Continuous stirred tank reactor

DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene

DMF: Dimethylformamide

F: Flow-rate of carrier gas

FTIR: Fourier-transform infrared

IL: Ionic liquid

IL-1: 1-N-butyl-3-methylimidazolium hydrogen chloride

IL-2: 1-Butyl-4-methylpyridinuium bromide

IL-3: Trihexyl (tetradecyl) phosphonium bromide

IL-4: 1,3-Disulfonic acid imidazolium hydrogen sulfate

K: Kelvin

K: Adsorption constant

k: Rate constant

K₂CO₃: Potassium carbonate

KBr: Potassium bromide

K10: Montmorillonite clay

L: Lewis

La: Lanthanum

La₂(HPO₄)₃: Lanthanum hydrogen phosphate

LDH: Layered double hydroxides

Leu: Leucine

LHSV: Liquid hour space velocity

MgO: Magnesium oxide

NaNO₂: Sodium nitrite

PEG: Poly(ethylene glycol)

PFR: Plug flow reactor

PTC: Phase transfer catalyst

SAPO: Silicoaluminophosphate

Sb: Antimony

SiO₂: Silica

Sm: Samarium

Sr(HSO₄)₂: Strontium hydrogen sulfate

SrSO₄: Strontium sulfate

TPA: Tungustophosphoric acid

TPD: Temperature programmed desorption

TPO: Temperature programmed oxidation

TBAB: Tetrabutylammonium bromide

UV: Ultraviolet

WHSV: Weight hour space velocity

Xp: Conversion of phenol

Acknowledgments

Priyanka Bhongale acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi, India for Senior Research Fellowship. The authors thank Academy of Scientific & Innovative Research (AcSIR) and CSIR-National Chemical Laboratory for additional support.

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website.