Elsevier

Catalysis Today

Volume 331, 1 July 2019, Pages 35-42
Catalysis Today

Theoretical predictions on dehydrogenation of methanol over copper-silica catalyst in a membrane reactor

https://doi.org/10.1016/j.cattod.2017.11.023Get rights and content

Highlights

  • Dehydrogenation of methanol over Cu/SiO2 catalyst in a tubular reactor was studied.

  • Theoretical comparison of tubular and membrane reactors were provided.

  • Dehydrogenation process was conjugated with oxidation of removed hydrogen.

  • The highest methyl formate yield was achieved in a membrane reactor at 125 °C.

Abstract

Dehydrogenation of methanol was performed over copper-silica catalyst. Methyl formate decomposition to carbon monoxide and hydrogen was considered as a main side reaction. Tubular and membrane reactors were compared theoretically in terms of efficiency of the process. For this purpose, a two-dimensional non-isothermal stationary mathematical model of the catalytic membrane reactor was developed and applied. The reaction of methanol dehydrogenation (in a tube side) was conjugated with hydrogen oxidation reaction (in a shell side). Conjugation of the processes was found to increase the methanol conversion up to 87% and achieve the methyl formate yield as high as 80% at 125 °C. The impact of various parameters on the process characteristics was studied using the developed mathematical model.

Introduction

During the last decades, a problem of methanol production and usage remains one of the most important and high demanded tasks of modern industry. Methanol is considered as a promising universal energy carrier, which would help to solve the actual problems of energy, transportation, and ecology. Another industrially important approach involving methanol is the Methanol to Hydrocarbons process, which is used to convert it to products such as olefins and gasoline [1], [2]. In terms of C1-chemistry concept, methanol is used as a raw stock for obtaining a wide range of industrially important oxygen-containing compounds and products on their basis (formaldehyde, methyl acetate, acetic acid, dimethyl ether, etc.) [3].

Methyl formate, one of the perspective derivatives of methanol, is known to be attractive as a solvent for colloids, fats, fatty acids, cellulose acetate, and so on. On the other hand, it can be used to produce a large number of chemical compounds (formic, acetic, propionic acids and their esters, various formamides, etc.) [4], [5].

In industry, methyl formate is conventionally produced from methanol via carbonylation process [6]. An alternative approach to obtain methyl formate is dehydrogenation of methanol, which attracts attention due to its high efficiency and great potential of usability (mild process conditions, low sensitivity towards impurities in feedstock). In addition, hydrogen released as a second product of this reaction can be used in various chemical syntheses and as a fuel.

At the same time, the process of methanol dehydrogenation is disadvantageously characterized with low selectivity. It is well known that an increase of methanol conversion leads to diminished selectivity towards target products, while the side reactions, including methyl formate decomposition to carbon monoxide and hydrogen, take place. The contribution of the side reactions depends on the process conditions and type of the catalyst. Copper-containing catalysts were shown to be the most selective in this process [7], [8]. Their activity is mostly determined by the preparation method and nature of the support.

It should be emphasized that reaction of methanol dehydrogenation is thermodynamically limited; therefore high temperatures are required to improve the yield of the target product. An increase of the temperature raises the methyl formate content in the equilibrium mixture but the process of its decomposition also accelerates. On the other hand, conversion of methanol can be increased by shifting the equilibrium towards products due to removal of one of the products (hydrogen, for example) from the reaction zone. Modern membrane technologies provide such possibility [9]. The efficiency of hydrogen removal from the reaction zone using various membranes has been demonstrated for production of formaldehyde, which is another key intermediate derived from methanol [10], [11], [12], [13], [14], [15]. Thus, in a temperature range of 350–500 °C a significant increase of methanol conversion and formaldehyde yield (more than 20%) was achieved in the case of membrane reactor [11], [12]. Dehydrogenation of methanol to methyl formate under atmospheric pressure using the Cu- and Cu-Pd catalytic membrane reactors was reported in [16]. The values of methanol conversion and methyl formate yield have achieved 57.3% and 50.0% at 240 °C in the case of membrane reactor, while fixed-bed reactor gave values of 43.1% and 36.9%, respectively.

In order to selectively remove the hydrogen from the reaction zone, which is important for a number of the industrially important chemical processes, the dense Pd-based membranes are of a wide interest [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Among the dense bimetallic systems, the Pd-Ag alloys were shown to be the most efficient for both the hydrogen separation and hydrogen sensing purposes [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. The use of the Pd-Ag alloys allows one to increase the permeability of the dense membrane and improve its stability by preventing the hydrogen embrittlement phenomenon, which is known to take place at the temperatures below 300 °C [28]. On the other hand, a presence of other molecules might affect negatively the permeability of the Pd-based membrane. Thus, in the case of carbon monoxide, a competitive adsorption of CO and hydrogen on the membrane surface can occur. The hydrocarbons presented in the reaction mixture can form the coke deposited on the membrane surface, which blocks the surface and complicates the hydrogen flux through it [26]. In our case, the reaction mixture contains CO, whereas the hydrocarbon coking problem is not important. However, for the silver-doped Pd membranes the inhibition effect from CO is known to be negligible [22], [25], and thus could be untaken into account.

Mathematical modeling represents by itself a platform for the theoretical investigation and estimation of the efficiency of the industrially important processes. A considerable number of papers have been devoted to simulation of dehydrogenation processes in the catalytic membrane reactors. However, most of them are related to the dehydrogenation of various hydrocarbons [29], [30], [31], while alcohols dehydrogenation is considered rarely. All models described in the literature can be conventionally divided into one- and two-dimensional. Thus, one-dimensional mathematical models were used to describe the conjugate processes for dimethyl ether, methyl formate and hydrogen obtaining from methanol [32], [33], [34], synthesis and simultaneous processing of methanol to produce methyl formate [35], dehydration of methanol into dimethyl ether with simultaneous dehydrogenation of cyclohexane [36], and oxidative dehydrogenation of methanol to formaldehyde [37]. One-dimensional mathematical models reflect the general laws of the process only, while the radial heat and mass transfers are not taken into account. It significantly reduces the reliability and importance of the results obtained. The use of two-dimensional mathematical models, which take into consideration the mass transfer along the reactor length, the radial diffusion of components through the internal and/or external part of the reactor, and the membrane support, attracts much interest. In order to study the heat effects, especially in the case of conjugated exothermic and endothermic processes, non-isothermal models are of great importance. As an example, the non-isothermal two-dimensional model was reported in [38] for oxidative dehydrogenation of methanol. This model considers mass transfer of components through the pores of the composite membrane, but, unfortunately, heat and mass transfer in the radial direction in the inner and outer parts of tube was out of consideration. These processes, however, can have a significant influence on the dehydrogenation process in general, and their accounting is necessary. Thus, for simulation the dehydrogenation process in the catalytic membrane reactor it seems extremely important to use the two-dimensional non-isothermal mathematical models that take into account radial heat and mass transfers in the inner part of the tube and in the ceramic support, as well as the hydrogen flux through the membrane. Recently we have developed and reported the non-isothermal two-dimensional mathematical model of catalytic membrane reactor for the process of hydrocarbons dehydrogenation, which was thermodynamically coupled with oxidation of hydrogen permeated through the membrane [39]. In present work, the model was adapted to be applicable for studying the process of direct dehydrogenation of methanol over Cu/SiO2 catalyst. Kinetic parameters required for the mathematical modeling of the process were experimentally obtained recently [40]. Additionally, the effect of oxidation of hydrogen being removed from the reaction zone through dense palladium-silver alloy membrane on the methanol conversion and methyl formate yield in the catalytic membrane reactor has been theoretically explored. In order to obtain the highest methanol conversion along with appropriate selectivity towards methyl formate, a set of the process parameters was optimized.

Section snippets

Material and methods

The Cu/SiO2 catalyst was prepared by a conventional impregnation method [41]. Silica gel with a surface area ∼300 m2/g was impregnated with an aqueous solution of copper nitrate in order to obtain copper loading of 5 wt.%. Then, the sample was dried in air at 120 °C for 1 h and calcined in argon at 300 °C for 1 h. The reduction procedure was performed in hydrogen stream during 2 h at 200 °C.

Kinetic studies of methanol dehydrogenation were carried out in a conventional fixed bed reactor system working

Mathematical model of the catalytic membrane reactor

The scheme of catalytic membrane reactor is shown in Fig. 1. The membrane reactor consists of two concentric tubes. The interior ceramic tube is filled with the fixed bed Cu/SiO2 catalyst. A thin palladium-silver alloy is deposited on the outer surface of thermostable ceramic support. The space between the interior and exterior tubes of the reactor is filled with oxidation catalyst.

The simplifying assumptions for two-dimensional non-isothermal stationary mathematical model were reported earlier

Results and discussion

The reaction of methanol dehydrogenation was considered along with the side reaction of methyl formate decomposition in the tube compartment of the membrane reactor:2CH3OH  CH3OCHO + 2H2, ΔH298 = 52.6 kJ/mol (Reaction 1)CH3OCHO  2CO + 2H2, ΔH298 = 128.8 kJ/mol (Reaction 2)

Kinetic studies of Cu/SiO2 catalyst reported earlier [40] gave the following parameters for these reactions:w1=k+1(CCH3OH2CCH3OCHOCH22/Keq),k+1=k10eEa1/RT,k10=5.1106M1s1,Ea1=34.5kJ/mol

The reaction of methyl formate decomposition

Conclusions

A process of methanol dehydrogenation to methyl formate over silica-based copper catalysts was studied theoretically. The results of process simulation have shown the higher efficiency of the membrane reactor in comparison with the tubular one. The maximum value of methanol conversion was obtained in the case of additional hydrogen oxidation in the shell side of the membrane reactor. The influence of certain relevant parameters, such as temperature, contact time, etc. on the process performance

Acknowledgments

This work was supported by Russian Academy of Sciences and Federal Agency of Scientific Organizations (state-guaranteed order for BIC, project number 0303-2016-0014). The numerical calculations were carried out at Tomsk Polytechnic University within the framework of Tomsk Polytechnic University Competitiveness Enhancement Program (grant VIU-TOVPM-316/2017).

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