Methyl methacrylate (MMA) is an important organic chemical raw material and polymer monomer, mainly used in the production of organic glass, molding plastics, acrylics, coatings and pharmaceutical functional polymer materials, etc. It is a high-end material for aerospace, electronic information, optical fiber, robotics and other fields.

As a material monomer, MMA is mainly used in the production of polymethyl methacrylate (commonly known as plexiglass, PMMA), and can also be copolymerized with other vinyl compounds to obtain products with different properties, such as for the manufacture of polyvinyl chloride (PVC) additives ACR, MBS and as a second monomer in the production of acrylics.

At present, there are three types of mature processes for the production of MMA at home and abroad: methacrylamide hydrolysis esterification route (acetone cyanohydrin method and methacrylonitrile method), isobutylene oxidation route (Mitsubishi process and Asahi Kasei process) and ethylene carbonyl synthesis route (BASF method and Lucite Alpha method).

1、Methacrylamide hydrolysis esterification route
This route is the traditional MMA production method, including the acetone cyanohydrin method and the methacrylonitrile method, both after the methacrylamide intermediate hydrolysis, esterification synthesis of MMA.

 (1) Acetone cyanohydrin method (ACH method)

The ACH method, first developed by the U.S. Lucite, is the earliest industrial production method of MMA, and is also the mainstream MMA production process in the world at present. This method uses acetone, hydrocyanic acid, sulfuric acid and methanol as raw materials, and the reaction steps include: cyanohydrinization reaction, amidation reaction and hydrolysis esterification reaction.

The ACH process is technically mature, but has the following serious disadvantages:

○ The use of highly toxic hydrocyanic acid, which requires strict protective measures during storage, transportation and use;

○ By-production of a large amount of acid residue (aqueous solution with sulfuric acid and ammonium bisulfate as main components and containing a small amount of organic matter), the amount of which is 2.5~3.5 times that of MMA, and is a serious source of environmental pollution;

o Due to the use of sulfuric acid, anti-corrosion equipment is required, and the construction of the device is expensive.

 (2) Methacrylonitrile method (MAN method)

Asahi Kasei has developed the methacrylonitrile (MAN) process based on the ACH route, i.e., isobutylene or tert-butanol is oxidized by ammonia to obtain MAN, which reacts with sulfuric acid to produce methacrylamide, which then reacts with sulfuric acid and methanol to produce MMA. the MAN route includes ammonia oxidation reaction, amidation reaction and hydrolysis esterification reaction, and can use most of the equipment of the ACH plant. The hydrolysis reaction uses excess sulfuric acid, and the yield of intermediate methacrylamide is almost 100%. However, the method has highly toxic hydrocyanic acid by-products, hydrocyanic acid and sulfuric acid are very corrosive, the reaction equipment requirements are very high, while the environmental hazards are very high.

2、 Isobutylene oxidation route
Isobutylene oxidation has been the preferred technology route for major companies in the world because of its high efficiency and environmental protection, but its technical threshold is high, and only Japan once had the technology in the world and blocked the technology to China. The method includes two kinds of Mitsubishi process and Asahi Kasei process.

 (1) Mitsubishi process (isobutylene three-step method)

Japan’s Mitsubishi Rayon developed a new process to produce MMA from isobutylene or tert-butanol as raw material, two-step selective oxidation by air to get methacrylic acid (MAA), and then esterified with methanol. After the industrialization of Mitsubishi Rayon, Japan Asahi Kasei Company, Japan Kyoto Monomer Company, Korea Lucky Company, etc. have realized industrialization one after another. The domestic Shanghai Huayi Group Company invested a lot of human and financial resources, and after 15 years of continuous and unremitting efforts of two generations, it successfully developed independently the two-step oxidation and esterification of isobutylene clean production MMA technology, and in December 2017, it completed and put into operation a 50,000-ton MMA industrial plant in its joint venture company Dongming Huayi Yuhuang located in Heze, Shandong Province, breaking the technology monopoly of Japan and becoming the only company with this technology in China. technology, also making China the second country to have the industrialized technology for the production of MAA and MMA by oxidation of isobutylene.

 (2) Asahi Kasei process (isobutylene two-step process)

Japan’s Asahi Kasei Corporation has long been committed to the development of direct esterification method for the production of MMA, which was successfully developed and put into operation in 1999 with a 60,000-ton industrial plant in Kawasaki, Japan, and later expanded to 100,000 tons. The technical route consists of a two-step reaction, i.e. the oxidation of isobutylene or tert-butanol in the gas phase under the action of Mo-Bi composite oxide catalyst to produce methacrolein (MAL), followed by the oxidative esterification of MAL in the liquid phase under the action of Pd-Pb catalyst to produce MMA directly, where the oxidative esterification of MAL is the key step in this route to produce MMA. The Asahi Kasei process method is simple, with only two steps of reaction and only water as a by-product, which is green and environmentally friendly, but the design and preparation of the catalyst is very demanding. It is reported that Asahi Kasei’s oxidative esterification catalyst has been upgraded from the first generation of Pd-Pb to the new generation of Au-Ni catalyst.

After the industrialization of Asahi Kasei technology, from 2003 to 2008, domestic research institutions started a research boom in this area, with several units such as Hebei Normal University, Institute of Process Engineering, Chinese Academy of Sciences, Tianjin University and Harbin Engineering University focusing on the development and improvement of Pd-Pb catalysts, etc. After 2015, domestic research on Au-Ni catalysts started Another round of boom, representative of which is Dalian Institute of Chemical Engineering, Chinese Academy of Sciences, has made great progress in the small pilot study, completed the optimization of nano-gold catalyst preparation process, reaction condition screening and vertical upgrade long-cycle operation evaluation test, and is now actively cooperating with enterprises to develop industrialization technology.

3、Ethylene carbonyl synthesis route
The technology of ethylene carbonyl synthesis route industrialization includes BASF process and ethylene-propionic acid methyl ester process.

 (1) ethylene-propionic acid method (BASF process)

The process consists of four steps: ethylene is hydroformylated to obtain propionaldehyde, propionaldehyde is condensed with formaldehyde to produce MAL, MAL is air oxidized in a tubular fixed-bed reactor to produce MAA, and MAA is separated and purified to produce MMA by esterification with methanol. The reaction is the key step. The process requires four steps, which is relatively cumbersome and requires high equipment and high investment cost, while the advantage is the low cost of raw materials.

Domestic breakthroughs have also been made in the technology development of ethylene-propylene-formaldehyde synthesis of MMA. 2017, Shanghai Huayi Group Company, in cooperation with Nanjing NOAO New Materials Company and Tianjin University, completed a pilot test of 1,000 tons of propylene-formaldehyde condensation with formaldehyde to methacrolein and the development of a process package for a 90,000-ton industrial plant. In addition, the Institute of Process Engineering of the Chinese Academy of Sciences, in cooperation with Henan Energy and Chemical Group, completed a 1,000-ton industrial pilot plant and successfully achieved stable operation in 2018.

 (2) Ethylene-methyl propionate process (Lucite Alpha process)

Lucite Alpha process operating conditions are mild, product yield is high, plant investment and raw material costs are low, and the scale of a single unit is easy to do large, currently only Lucite has exclusive control of this technology in the world and is not transferred to the outside world.

The Alpha process is divided into two steps:

The first step is the reaction of ethylene with CO and methanol to produce methyl propionate

using palladium-based homogeneous carbonylation catalyst, which has the characteristics of high activity, high selectivity (99.9%) and long service life, and the reaction is carried out under mild conditions, which is less corrosive to the device and reduces the construction capital investment;

The second step is the reaction of methyl propionate with formaldehyde to form MMA

A proprietary multi-phase catalyst is used, which has high MMA selectivity. In recent years, domestic enterprises have invested great enthusiasm in the technology development of methyl propionate and formaldehyde condensation to MMA, and have made great progress in catalyst and fixed-bed reaction process development, but the catalyst life has not yet reached the requirements for industrial applications.