Towards an industrial synthesis of diamino diphenyl methane (DADPM) using novel delaminated materials: A breakthrough step in the production of isocyanates for polyurethanes

Abstract

Delaminated materials ITQ-2, ITQ-6 and ITQ-18 are very efficient catalysts of zeolitic nature for the synthesis of diamino diphenyl methane (DADPM), the polyamine precursor in the production of MDI for polyurethanes. The exfoliation process results in excellent accessibility of their active sites to reactant molecules as well as fast desorption of products. These catalysts present higher activity and slower rates of deactivation than their corresponding zeolites. Moreover, the topology of the delaminated structure imposes a precise control of the isomer distribution, offering an additional flexibility in the synthesis of DADPM. By optimizing the process conditions it is possible to achieve final DADPM crude under industrial production specifications with ITQ-18. This catalyst represents a real chance for replacing HCl in the industrial production of DADPM.

Introduction

Diamino diphenyl methane (DADPM, also named methylenedianiline, MDA) is the precursor of MDI (methylene diphenylene diisocyanate), an important monomer for the synthesis of polyurethanes and thermoplastics. Polyurethanes are used in the production of flexible and rigid foam products, coatings, adhesives, sealants, elastomers and binders, which have application in a wide variety of industries like automotive, footwear, refrigeration, construction and furniture [1]. In the USA alone, the 2003 production of polyurethanes and related products was 4.9 million tonnes, with a 2.8% growth increase over 2002 [2].

Commercially, DADPM is obtained by acid catalyzed condensation of 2 molecules of aniline with 1 molecule of formaldehyde at 60–80 °C. The reaction mixture is then heated at 100–160 °C to complete the reaction. A sub-stoichiometric quantity of hydrochloric acid is normally used to catalyze the process, which after the condensation has to be neutralized with NaOH giving an equivalent amount of NaCl. The final mixture separates in two layers, organic and aqueous. The former phase is distilled to recover the unreacted aniline and crude DADPM is obtained [3], [4]. This usually contains more than 60% of 4,4′-DADPM (the para-isomer) and small amounts (3–5%) of the other diamines, 2,4′-DADPM and 2,2′-DADPM (named ortho-isomers). Besides DADPM, higher molecular weight species (basically triamines and tetramines) are also present in the product mixture (20–25%). The product distribution is strongly influenced by the process variables and a high quality 4,4′-DADPM crude is preferentially obtained by working at mild temperatures and high aniline to formaldehyde (A/F) molar ratio. The recovered DADPM product may then be used to produce MDI by reaction with phosgene or conversion to isocyanate by non-phosgene routes.

The main drawback of the homogenous process is the disposal of the large amount of salt water liberated in the neutralization of HCl that contains traces of aromatic amines which have to be removed by biological treatment before discharging the residual water.

Substitution of HCl by a suitable solid acid will minimize the generation of polluting water and it could allow the re-use of the catalyst, resulting in a simpler process [3]. Many solid acid catalysts have been evaluated over the past 30 years for the synthesis of DADPM, as recently revisited by de Angelis et al. [4]. Among them, ion-exchange resins have shown excellent selectivity to the para-isomer, although their specific activity is low and the final recovery and regeneration of the used catalyst has not been solved [5], [6]. Very recently the researchers from Dow Global Technologies Inc. have obtained high yield in the production of 4,4′-DADPM and its polymeric derivatives over an ion exchange resin based in a styrene–divinylbenzene co-polymer but, unfortunately, this system presents the same serious limitations for the re-use of the expended catalyst [7].

Different salts of tungsten have also been tested (borides, silicides and sulfides) [8], [9]. They present very mild acidity and their activities are low even at high reaction temperatures (200 °C). Acidic clays [10], [11], [12], [13] and diatomaceous earth [12] have also shown moderate activity for this process, which then requires high reaction temperatures, favoring ortho-substitution and the formation of polymeric DADPM. Silica–alumina has been claimed as an active catalyst for DADPM synthesis [14], [15], [16], but also with low yields of primary amines. However, recently Perego et al. have proposed the use of several amorphous aluminosilicates characterized by a narrow pore size distribution in the mesopore region [17], [18], [19]. These materials show high activity in the conversion of an aniline–formaldehyde condensate (aminal) to DADPM with a very high ratio 4,4′-DADPM/(2,2′ + 2,4′-DADPM) (5.99–6.71). Unfortunately, they exhibit a relatively short catalyst life and some partial destruction of the pore structure during thermal regeneration.

Despite the results obtained with other inorganic solid acids, the best performance ever achieved in DADPM production used zeolitic materials. Zeolite Y pre-treated with a fluorinated agent [20], and Beta zeolite [17], [21], [22], [23], [24], [25] in particular, have shown clear improvements in catalyzing the reaction. There are three main advantages of microporous materials as catalyst for this process: (i) high acid site density, thus, improving catalytic activity and catalyst life; (ii) a shape selectivity effect due to confined molecular diffusion. (The work of researchers from Eni Tecnologie is particularly relevant, as by modifying the pore aperture and channel diameter of Beta zeolite through a silylation treatment, they were able to increase the ratio 4,4′-DADPM/(2,2′ + 2,4′-DADPM) in the range 2.2–5.8 [4], [23]. A similar effect was also observed upon treatment of the zeolite with boric acid (H₃BO₃) or with orthophosphoric acid (H₃PO₄) as free acids or as ammonia salts [24]; (iii) a moderate water resistance is claimed for some zeolites, with a maximum tolerance of 3% H₂O in the feed.

The main drawback of zeolites is the strong limitation to molecular diffusion imposed by a structure built of narrow geometrical channels. Even in the case of a tri-dimensional zeolite with twelve-ring (12-ring) pores, e.g., Beta, the process is controlled by diffusion, which becomes evident as a reduction of the crystallite size gives a significant enhancement of the primary amines production [26]. Moreover, restrictions to diffusion of reactants and products can be even worse in the case of medium-pore size zeolites (e.g., ZSM-5, ERB-1) [21], [22] or unidimensional structures (e.g., SSZ-59) [26].

As has been shown elsewhere, the accessibility of acid sites in zeolitic materials can be drastically enhanced by delamination of a given structure [27], [28], [29], [30]. The exfoliation process can produce single layered structures with large external surface areas (≥300 m² g⁻¹) and very little, if any, microporosity. As a consequence, active sites are highly accessible through the external surface and, at the same time, reactant diffusion and product desorption are faster, as the process takes place basically on the outer shell [31]. This produces catalysts that are more active than their corresponding zeolites for reactions controlled by the diffusion into the micropores. On this basis, several delaminated zeolitic materials have been proposed as suitable catalysts for the synthesis of DADPM [26], with higher performance than large pore zeolites and with a much lower rate of deactivation and superior resistance to water content in the feed (up to 5%). Nevertheless, the amines distribution seems to be strongly affected both by the process conditions and the topology of the delaminated material. In the present contribution we explore the keys for the efficient preparation of DADPM with delaminated materials ITQ-2, ITQ-6 and ITQ-18. After presenting the reaction scheme corresponding to a process catalyzed by solid acids, the catalytic behavior of delaminated materials will be compared with their corresponding zeolite precursors. Furthermore, we provide optimized process conditions to accomplish the manufacture of crude DADPM to industrial production specifications, both in batch and continuous flow reactor systems.