Ind. Eng. Chem. Res. 2001, 40, 4157-4161
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Personal Perspective of the Development of Para Selective ZSM-5 Catalysts Nai Y. Chen† Technical Consultant, 4 Forrest Central Drive, Titusville, New Jersey 08560-1310
The evolution of para selective catalysis from a concept to a commercial process for the para selective synthesis of xylene took more than 17 years. While the mechanism of molecular selectivity was established both experimentally and mathematically shortly after the proof of its existence, the development of a new process faced a number of unexpected hurdles. Described herein is a summary of the scientific achievements and the managerial decisions made over the years that eventually led to the commercialization of this process. It demonstrates the importance of interdisciplinary participation in industrial research and the benefits of exploring new applications of heterogeneous catalysts outside the traditional research of a petroleum refining industry. Introduction The selective production of para-directed aromatics has gained its important position in the petrochemical industry since 1988 by the commercialization of MSTDP (Mobil’s selective toluene disproportionation process).1 Para selectivity was first demonstrated in the laboratory by the alkylation of toluene with methanol over ZSM-5 in 1971.2 The yield of p-xylene was found to exceed the maximum concentration limited by the law of chemical equilibrium. This finding corroborated with an earlier finding with a small-pore (containing eight-membered oxygen rings in its structure) zeolite, when the rate of hydrogenation of trans-1, 3-piperylene and transbutene-2 was found to be much faster than that of their cis isomers.3 Both findings supported the hypothesis that the rate of intracrystalline transport could affect the reaction kinetics and product distribution. This hypothesis was later rationalized mathematically by James Wei in 1982.4 The development and commercialization of the first process took a tortuous path of 17 years. We discuss here some of the unique features along the tortuous path of the development of a novel chemical process initiated in the petroleum refining research laboratory and penetrated into the petrochemical industry. Background A. Zeolite Catalysis. Natural zeolites had been known for many years to possess molecular sieving properties; i.e., they can be used to separate straightchain molecules from branched-chain molecules. However, the synthesis of new zeolites, pioneered by Union Carbide, was a monumental technical achievement.5,6 One of the excitements in zeolite catalysis was the discovery of their superactivity as cited by Weisz and Frilette in their 1960 paper.7 “The zeolite activity is attributed to its crystalline structuressuch regularity in a solid. It gives us the vision of the ability to identify and describe a catalytic site in such quantitative terms.” †
Retired Senior Scientist and Research Advisor, Mobil Research & Development Corp. Telephone: (609)737-0321. Fax: (609)737-8206. E-mail:
[email protected].
They also coined the term “shape selective catalysis” to describe the selective cracking of aliphatic hydrocarbons.8 In addition to catalytic cracking, shape selective oxidation and hydrogenation were demonstrated by using an encapsulated Pt/4A (sodium form) and Pt/5A (calcium form) catalyst in the early 1960s.9,10 While the selective hydrogenation of n-olefins and n-diolefins in the presence of their branched isomers was expected, the selectivity among the straight-chain isomers was totally unexpected.3 It remained as a curiosity for many years. The application of zeolites as catalysts in aromatics reactions such as benzene alkylation and xylene isomerization also attracted considerable attention in the early 1960s.11,12 B. ZSM-5. On January 19, 1967, familiar with my studies on shape selective catalysis, Bob Landolt came to ask me whether I would be interested in testing a new zeolite, ZSM-5, which had the unusual characteristics of sorbing more hydrocarbon molecules than water and more n-hexane than cyclohexane. He told me that, back in 1963, he and Bob Argauer13 discovered this zeolite by using tetrapropylammonium hydroxide in its synthesis following George Kerr’s pioneering studies using various tetraalkylammonium hydroxides in zeolite synthesis.14,15 On February 7, 1967, I received 5 g of ZSM-5 from him. It was a day of considerable significance in the history of shape selective catalysis. It was the day that ushered in an new era of shape selective catalysis, now known as catalysis by medium-pore zeolites, notably that of ZSM-5. These highly siliceous hydrophobic medium-pore zeolites have dominated the catalysis scene for more than 30 years. However, it had not received much attention because its unique hydrophobicity16,17 and shape selective catalytic properties18 were not identified until after 1967. Between 1967 and 1969, the Technology Exploration Group, led by me at Mobil, identified a number of reactions of commercial potential using ZSM-5, including catalytic hydrodewaxing of fuels (MDDW)19,20 and lubricating oil basestocks (MLDW);21,22 selective upgrading of catalytic naphtha reformate, known later as “M(obil)-Forming”;23,24 and aromatization of light hydrocarbons, known as M2-Forming.25,26
10.1021/ie000870p CCC: $20.00 © 2001 American Chemical Society Published on Web 05/18/2001
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In the meantime, another Exploration Group at Mobil Chemical started in late 1968 to scope the application of high silica hydrophobic zeolites for a variety of organic reactions.27 They studied the interaction of toluene and phenol or methyl benzoate and the rearrangement of methyl benzoate; the methyl group transfer among polymethylbenzenes, methanol, dimethyl ether, etc., led to the production of terephthalic acid and dimethyl terephthalate, precursors in the synthesis of polyesters and other polymers. Although they all received management attention, there was a lack of sufficient economic incentive for Mobil to commercialize the costly synthesis of ZSM-5. At the time, not knowing the crystalline structure of ZSM-5 other than its sorption properties, an alternative approach, i.e., to simulate its catalytic properties by blending different natural and synthetic zeolites, was attempted but failed. C. The Coming of “Octane Race”. The federal government decision in the late 1960s to remove the lead additives from gasolines brought a challenge to the oil industry to match the octant rating of their gasoline products by catalytic refining in place of the lead additives. The selective upgrading of catalytic naphtha reformate over ZSM-5,23,24 if developed, could be an attractive approach to meet this challenge. Pilot-plant data on commercial reformates showed that the improvement in octane rating was greater than the older Selectoforming process.28,29 Furthermore, when compared to conventional reformates at the same octane rating, the M-Forming products were low in benzene and toluene and high in C8 plus aromatics. M-Forming, thus, if commercialized, not only would meet the octane requirement without the lead additive but also would reduce the concentration of benzene in gasolines to below the specification that was imposed because of benzene’s toxicological hazards. In line with the economic incentive of this process, a decision was made by the laboratory management to develop this process in early 1970. A catalyst development program was immediately initiated to develop a commercial process to synthesize ZSM-5. A process development program soon followed in the summer of 1970. This effort was considered a crowning achievement in Mobil’s history in terms of the speed of development. The term “M(obil)-Forming” was coined to describe this process. The process was tested commercially. The start of the commercial test at its Frontignan Refinery in France was in late 1971. Unfortunately, the “octane race” did not materialize as anticipated by Mobil. The automobile industry and the oil industry jointly took an approach economically beneficial to both industries that would not require large capital expenditures to comply with federal regulations. The auto industry lowered their cost of manufacture by building internal combustion engines operating at lower compression ratios without the benefit of the lead additives; the oil industry would sell more lead free gasolines of lower octane ratings. However, the economic and environmental impacts to the society caused by the increase in energy consumption and the increased fuel cost to the consumers did not raise any societal or federal concerns. Looking back, although M-Forming did not achieve its promised potential, the decision to synthesize ZSM-5 on a commercial scale turned out to be an excellent move by Mobil. With the availability of ZSM-5, not only were
the distillate and lube dewaxing processes developed and commercialized but also it made possible the development of a long line of new processes. In fact, Mobil’s R&D program rode on the crest of this development for the next 25 years. Para Selectivity A. Early Attempt. The alkylation of toluene with methanol over HZSM-5 was studied at Mobil Research in as early as 1970.2 The initial data did not show any difference in the isomer distribution of the xylenes from the published data using catalysts such as AlCl3 and the amorphous silica alumina. Furthermore, it was disappointing to find that the transfer of the methyl group in methanol to toluene was very inefficient. Material balance calculations showed that 20-60% of the methanol in the feed was directly converted to C2C5 hydrocarbons. Because methanol was quite expensive as an alkylation agent, the conversion of methanol to hydrocarbons during alkylation was undesirable. Again, this situation changed dramatically by the Arab oil embargo in 1973. With the threat of a shortage of crude oil, if methanol could be economically converted to the traditional transportation fuels, its production from coal and natural gas would be a logical answer to the shortage of crude oil. Research into this approach attracted center stage attention by the mid-1970s for a number of years.30 B. Confirmation of Para Selectivity. With ZSM-5 of different crystal sizes provided by Mobil’s catalyst development program, a study of the gas chromatographic analysis of xylene mixtures using ZSM-5 as the gas chromatography (GC) column support was undertaken, and the results showed that the crystal size of ZSM-5 had a decided effect on the chromatographic retention time of the xylene isomers, supporting the concept that the rate of intracrystalline transport could affect the reaction kinetics and product distribution. During the summer of 1971, the study of the alkylation of toluene with methanol was repeated using large crystal ZSM-5. To deactivate external acid sites, the HZSM-5 samples were either treated with a bulky nitrogen compound such as phenyl carbazole or silylated with organic silicone compounds, such as dimethyldicholorosilane.31 Some of the early data are shown in Table 1. These experiments established the existence of “para selectivity” for the first time. However, the project was dropped again for the lack of commercial interest. It remained a laboratory curiosity until the end of 1972, when Warren Kaeding of Mobil Chemical, impressed by the data, reactivated this project in his laboratory. C. Extended Exploratory Research. Keading, an organic chemist, took a different approach in his effort to achieve para selectivity.32 Instead of selectivating large ZSM-5 crystals with a silicon compound, his group was able to alter the para selectivity of small crystal HZSM-5 and HZSM-11 by “chemical” modifications, involving the deposition of oxides of phosphorus, boron, magnesium, etc.33,34 A joint paper on this subject was published in J. Am. Chem. Soc. in 1979.35 Kaeding’s group further extended para selective reactions to the disproportionation of toluene to produce p-xylene,36,37 the ethylation of toluene to pethyltoluene,38 and the selective production of p-dialkylbenzenes in general.39
Ind. Eng. Chem. Res., Vol. 40, No. 20, 2001 4159 Table 1. Experimental Data on Methylation of Toluene over HZSM-5a crystal size, µm pretreatment WHSV temperature, °C % toluene methylated % xylene as para meta ortho a
