Industrial Production of Formaldehyde Using Polycrystalline Silver

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Industrial Production of Formaldehyde using Polycrystalline Silver Catalyst Graeme John Millar, and Mary Collins Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02388 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Industrial & Engineering Chemistry Research

Industrial Production of Formaldehyde using Polycrystalline Silver Catalyst *Graeme J. Millar and 1Mary Collins

Institute for Future Environments, and 1School of Chemistry, Physics & Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology (QUT), Brisbane, Queensland 4000, Australia. Email: [email protected]

We report the first comprehensive review of the industrial process for the catalytic partial oxidation of methanol to formaldehyde using silver catalyst. Formaldehyde remains a key, large volume industrial chemical which is ubiquitous to household, commercial, aviation, pharmaceutical and automotive products due to its high reactivity and versatility. Critical analysis of the various technological approaches used to manufacture formaldehyde, characteristics of silver catalysts employed, process conditions, and performance values has been presented. The three common plant types are: almost complete conversion (BASF type); incomplete conversion followed by distillation (ICI type); and the off-gas recycle process which dominates world production due to its extensive use in China. Polycrystalline silver remains the most popular catalyst, with only minimal use of alternate supported catalysts or gauze materials. Catalyst physical properties such as bulk density, purity, mechanical strength, size fraction and shape are important in relation to overall formaldehyde synthesis performance. Process yields majorly range from ca. 82 to 92 % and catalyst lifetimes vary from 4 weeks to 12 months. There is a research need to not only raise overall yields consistently to in excess of 90 % but to also maximise catalyst life. Aspects which should be taken into account include: purification of feed components; development of a systematic means for catalyst bed design; improvement and standardization of process configuration; equipment optimization; investigation of the benefits of feed or catalyst additives; protection of the catalyst bed from poisoning events and discovery of preferred process operating conditions. In the near term, focussing on optimization of the current formaldehyde synthesis industry is seen as more prospective from a commercial point of view, than longer term investigation of alternate pathways for manufacturing formaldehyde from methane or carbon dioxide which may not be viable for at least a decade.

ACS Paragon Plus Environment

1


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Key Words: silver catalyst; methanol; oxidation; formaldehyde; engineering; chemistry 1.

Introduction

Formaldehyde remains one of the most important chemicals produced in the world today due to its unparalleled reactivity and versatility 1. Primarily, formaldehyde is employed in the production of resins for the wood panel industry, with urea formaldehyde (UF), phenol formaldehyde (PF) and melamine formaldehyde (MF) resins and their variants representing the major formulations 2-5. Formaldehyde is also used to make chemicals such as polyacetal or polyoxymethylene (POM) 6, pentaerythritol 7, 8, paraformaldehyde 9, acetylenic chemicals 10

, hexamine

11

and methylal (dimethoxymethane)

12-14

. Consequently a wide range of

industrial applications depend upon formaldehyde such as construction, automotive, aviation, pharmaceuticals and cosmetics 1.

In the early twentieth century the manufacture of formaldehyde on a small scale was initiated in Europe and the USA 15. The development of high-pressure synthesis of methanol by BASF in 1923 allowed the production of formaldehyde on a true industrial scale

16

.

Initially, gauze made of silver wire was used as the catalyst element 17. However this type of catalyst has almost entirely replaced by a shallow catalyst bed of silver crystals 18. Further development resulted in the mixed iron oxide-molybdenum oxide catalyst

19

which now

competes with the silver process, accounting for approximately forty-five percent of the world’s production of formaldehyde

20

.

Other processes that employ oxidation of

hydrocarbons or hydrogenation of carbon monoxide are not industrially significant since they compare unfavourably with methanol conversion either in terms of cost or decreased yield 21, 22.

A concentration of 37 wt % formaldehyde is used as a standard for pricing or production data, although in reality industrial plants typically make formaldehyde solutions of up to 55 wt % concentration 1. The silver catalyst based process for formaldehyde manufacture proceeds by two pathways, a partial oxidation reaction [Equation 1] and a dehydrogenation reaction [Equation 2]. Equation 1:

+ → +

Equation 2:

→ +


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Despite the considerable importance of formaldehyde to our daily lives until this point there has not been a comprehensive review of the industrial synthesis using polycrystalline silver catalyst. Despite lower yields of formaldehyde compared to the metal oxide process the silver process continues to grow in popularity. This latter situation is at least in part due to inherently cheaper production plants as the ability to operate with excess methanol results in substantially smaller units than the metal oxide process which requires excess air. In addition, catalyst costs are normally less with silver compared to metal oxide and silver catalysts can be regenerated on site, which is not the case with the oxide materials. Utilities consumption is also less with silver based formaldehyde plants due primarily to the reduced need for electricity to drive the air blowers.

Consequently, this study will present

information regarding the industrial practices employed, silver catalyst properties, process engineering and operating principles. Ultimately, this review will provide an insight into the challenges to optimise formaldehyde synthesis and produce the next generation of formaldehyde manufacturing technology.

2.

Process Definitions

The formaldehyde industry employs a variety of terms to describe the efficiency of the methanol oxidation process. These terms are quantifiable measurements of the process efficiency and include methanol conversion, selectivity, formaldehyde yield, production ratio and catalyst life.

2.1

Selectivity

When converting the methanol feed it is important to produce valuable formaldehyde and not by-products such as carbon dioxide, carbon monoxide, and formic acid. Formaldehyde selectivity is a measure of the amount of formaldehyde produced per unit of methanol reacted and is defined as in Equation 3.

Equation 3:

! "#$%&'(& )#&*+& ×-..

% ℎ =

! /'%0 102 ! /'%0 3*/



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Also of interest is an understanding of the quantities of carbon dioxide and carbon monoxide as they give an indication of the extent of methanol combustion and formaldehyde decomposition during the formaldehyde synthesis process [Equations 4 & 5].

Equation 4:

! 8%#90 :; 5 = B(KK < -..EF)G-LH 7100 = 75.8%


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2.3

Formaldehyde Yield

Formaldehyde yield can be defined as shown in Equation 9.

Equation 9: % = ℎ P =

% "#$%&'(& Q+/;R;/( < % /'%0 80R# ;0 -..

Yield is the most often quoted parameter in formaldehyde plants and can range from 82 to 92%, depending on the quality of the plant operation. Even a one per cent change in yield can represent significant financial benefit to formaldehyde producers.

Many formaldehyde plants use “production ratio” as the key performance indicator which is defined as “kg of methanol required to make 1 tonne of 37 wt% formaldehyde”. The theoretical minimum value is 394.7 kg of methanol per tonne of 37 wt% formaldehyde. For formaldehyde plants which produce high concentration formaldehyde solutions, the production ratio is also defined as “kg of methanol required to make 1 tonne of 50 wt% formaldehyde”, which in turn means the theoretical minimum methanol consumption is 533.3 kg of methanol per tonne of 50 wt% formaldehyde. Finally, the Indian formaldehyde marketplace favours calculation of the production ratio in terms of “tonnes of 37 wt% formaldehyde made from 1 tonne of methanol”, with theoretical limit of 2.53 tonnes of 37 wt% formaldehyde. A summary of the relationship between the various yield definitions is illustrated in Table 1.

Table 1: Demonstration of the relationship between formaldehyde yield and production ratios Formaldehyde Yield

Production Ratio (kg

Production Ratio (kg

Production Ratio

(%)

of MeOH to make 1

of MeOH to make 1

(tonnes of 37 wt% FA

tonne of 37 wt% FA)

tonne of 50 wt% FA)

made from 1000 kg of MeOH)

82

481

650

2.08

83

476

643

2.10



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84

470

635

2.13

85

464

627

2.15

86

459

620

2.18

87

454

613

2.20

88

449

606

2.23

89

443

599

2.25

90

439

593

2.28

91

434

586

2.31

92

429

580

2.33

Table 2 displays an example of operating data from a formaldehyde plant using silver catalysts along with appropriate calculation of performance parameters.


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Table 2: Example of performance values from an operating formaldehyde plant Methanol Consumption

30,000

kg/day

Formaldehyde Produced

50,824

kg/day

Formaldehyde in Product

48

wt %

Methanol in Product

1.5

wt %

Formaldehyde Produced

65,934

37 wt% basis/day

Production Ratio

455

kg of methanol per tonne 37wt% formaldehyde

Formaldehyde Selectivity

89.24

%

Methanol Conversion

97.15

%

Formaldehyde Yield

86.7

%

2.4

Catalyst Life

Silver catalysts typically operate in a formaldehyde plant for a period of between a few months and a year. Silver sinters at the high reaction temperatures, thus resulting in elevation of pressure drop over the bed and usually decreased performance. It is preferable for catalysts to last as long as possible, however, there is a point where the lower productivity, higher power costs and safety considerations outweigh the cost of replacing the catalyst. The life of silver catalyst is highly dependent upon the operating conditions of the formaldehyde reactor. For example, the catalyst life is usually longer when the catalyst is operating at relatively low temperatures and comparatively small plant loads. The form of the silver catalyst used has also been observed to play a critical role in terms of provision of longer useful operating life as can be seen in Table 3.

Table 3: Examples of catalyst life for two types of silver catalyst in a range of formaldehyde plants Silver Catalyst Type 1 Life (days)

Silver Catalyst Type 2 Life (days)

Plant 1

180

100

Plant 2

100

75

Plant 3

43

23

Plant 4

110

42



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It has been proposed that the water ballast process (BASF) promotes longer catalyst life as introduction of water with its associated high heat capacity promotes homogenous distribution of heat over the catalyst bed and thus minimises sintering and coke formation 23

. The consequences of catalyst poisoning cannot be ignored and may be a major factor in

relation to catalyst life. A catalyst with a design life of six months can be significantly compromised due to the quality of methanol

24

. If the methanol feedstock has a high

permanganate reactivity the catalyst life was found to be reduced to only two months.

At the end of its useful life, silver catalyst is normally returned to the catalyst supplier for reprocessing.

Therefore, essentially the silver bullion component is loaned to the

formaldehyde producer and fabrication costs are incurred for the conversion of the silver to the catalyst form. Some formaldehyde producers have suggested that only the uppermost section of the silver bed should be removed and replaced with fresh silver catalyst

25

.

However, it has not been demonstrated that this latter approach is beneficial in terms of maintaining overall catalyst performance since the lower portion of the bed may have sintered or been poisoned during the previous operational campaign.

3.

Industrial Process Designs for Methanol Oxidation to Formaldehyde using Silver

Catalysts 3.1

Almost Complete Conversion of Methanol

Gerloff 26 described the single pass, almost complete conversion of methanol method which is often referred to as the “BASF process”.

Sperber

27

intimated that the operating

temperature of the silver catalyst was in the range 600 to 700 oC, and that formaldehyde yields were as high as 87.5 %. A mixture of air, steam and methanol was supplied to the silver catalyst in the gas phase, with the methanol/water mass ratio typically 60/40. Hence, why the BASF process is also termed the water ballast method

23

. The process for the

almost complete conversion of methanol to formaldehyde is shown in Figure 1. A mixture of methanol and water is typically fed into the evaporator vessel wherein air is sparged into the solution. The gaseous feed mixture is subsequently superheated to ensure the presence of liquid is eliminated prior to being fed to the reactor where it flows through a 20-30 mm


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thick bed of silver crystals. A gas filter system may be present immediately prior to the reactor entrance as is usually a flame arrestor for safety reasons.

Figure 1: Almost complete conversion process for methanol oxidation to formaldehyde

The product formaldehyde gas is thermally unstable and thus rapid cooling of the gaseous mixture is achieved by passage through a waste heat boiler system containing pressurized water 28. The exit gas stream is normally in the range 120 to 200 oC and this is then passed into an absorption tower which contains appropriate packing material.

The high

concentration formaldehyde solution is withdrawn from the bottom of the column and the non-condensable gases (off-gas) exit from the final stage which is usually a bubble-capped system

28

. To aid the production of high concentration formaldehyde solutions the water

quantity required for the feed is often initially added to the top of the absorber column and then drained from the exit of the final stage

29

. This process modification has two main

benefits: (1) the volume of water added to the absorber is 200 to 500 % greater than in normal operation which means that the emissions of formaldehyde and methanol from the column are minimized; (2) the water contains recaptured methanol and formaldehyde which are recycled to the reactor and ultimately enhance process yield. An example of



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operating parameters for an almost complete conversion formaldehyde unit is shown in Table 4.

Table 4: Typical operating parameters for an almost complete conversion formaldehyde plant Air Flow Rate

6040

Nm3/h

Air Flow Rate

6,040,000

L/h

Molar Air Flow Rate

269643

mol/h

Methanol Flow Rate

4667

kg/h

Methanol Purity

99

%

Methanol Quantity

4620

kg/h

Molar Methanol Flow Rate

144375

mol/h

Molar Air/Methanol Ratio

1.87

Water Flow Rate

2364

kg/h

Water Purity

94

%

Actual Water Flow

2222

kg/h

Steam/Methanol Mass Ratio

0.48

3.2

Incomplete Conversion of Methanol Followed by Distillation

An alternative approach to formaldehyde synthesis is to operate the production unit with a large excess of methanol such that distillative recovery of methanol is required to meet product specifications [Figure 2]. This approach has the advantage that comparatively low reaction temperatures are often employed which may suppress undesirable secondary reactions. For example, Eek-Vancells reacted a methanol/air/steam mix at 560 oC to obtain an effluent comprising of 1000 kg/h formaldehyde, 782.2 kg/h methanol and 470 kg/h water (in addition to non-condensable products) 30. Distillation facilitated production of a solution containing 55 wt% formaldehyde and 5 =

-

TUQV

× 3600

The useful operating region for space velocity is typically 100,000 to 250,000 h-1. The precise space velocity which provides optimum process benefits depends upon the plant configuration. When the space velocity is less than the minimum recommended value then problems can occur due to side reactions which typically produce carbon deposits. Formaldehyde is not stable under normal reaction conditions and will decompose to form carbon monoxide in the gas phase. Carbon monoxide can undergo further changes via the Boudouart Reaction to make carbon and carbon dioxide.

Methanol loading on the catalyst bed is another useful measure when operating the formaldehyde plant. A typical value for catalyst loading is 2.0 metric tons of methanol per hour per m2 of catalyst bed area which equates to 25 kg/min/m2 86. Table 6 provides an illustration of the variation of methanol loads encountered in industrial formaldehyde plants.

Table 6: Examples of process conditions in a range of formaldehyde plants MeOH flow

Area of Bed Surface

Methanol Load

(kg/min)

(m2)

(kg/min/m2)

Plant 1

15.625

0.697

22.4

Plant 2

23.6

0.697

34

Plant 3

36.11

1.33

27.2

Plant 4

33.33

1.21

27.5

Plant 5

64.17

2.27

28

Plant 6

43.05

1.17

36.8

Plant 7

31.25

1.13

27.7

4.13.2 Feed Ratios Szustakowski 85 examined the importance of oxygen/methanol molar ratio during methanol oxidation on a silver catalyst. He found that both methanol conversion and formaldehyde yield were promoted when the O2/CH3OH ratio was increased from 0.25 to 0.40.



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Waterhouse et al. 87 found that a molar ratio of methanol to oxygen of 2.25 was optimal in terms of degree of methanol conversion and formaldehyde selectivity of polycrystalline silver catalyst. These latter authors ascribed the relationship between feed stoichiometry and catalyst performance in terms of the presence of various oxygen species on the silver catalyst. For example, at low CH3OH/O2 ratios the dominance of the non-selective Oα species was postulated. Lefferts et al. 88 noted that increasing oxygen concentration in the feed generally enhanced formaldehyde production while also promoting carbon monoxide and carbon dioxide formation. Use of higher reaction temperatures appeared to diminish the amount of carbon dioxide formed.

It has been recognised for some time that addition of water to the reactor feed improves conversion and yield. The oxygen concentration in the feed can be raised without causing overheating, because some of the reaction heat is absorbed by water. The higher oxygen concentration uses a larger proportion of hydrogen formed in the endothermic formaldehyde synthesis reaction, and thus favourably shifts the equilibrium. The addition of water can also activate the catalyst and extend the catalyst life. Lightly bound catalyst poisons are often stripped off at high water addition rate. The optimal yield usually occurs at a 60/40-weight ratio of methanol/water 86. Water fed to the reactor can be supplied as moisture in the air, as water in the methanol feed, or as a separate stream. Water to be added to the reactor feed can also be supplied be recycling dilute formaldehyde solution from the top section of the absorber.

4.13.3 Dehydrogenation/Oxidation ratio An insight to the formaldehyde plant operating conditions can be determined by calculation of the dehydrogenation/oxidation ratio which can be estimated as shown in Equation 11. Equation 11:

6ℎ Y5 5/ 7 5 = > [ =ℎ5 6ℎ Y5 ℎ = = > [ =ℎ5 7> ℎ Nagy and Mestl 89 proposed that surface embedded atomic oxygen (Oγ) was responsible for the direct dehydrogenation of methanol reaction to produce formaldehyde and hydrogen, and that surface adsorbed atomic oxygen (Oα) was responsible for the non-selective


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oxidation of formaldehyde. Examination of the hydrogen content of the product gas revealed that at elevated temperatures the direct dehydrogenation reaction dominates and this result was in accord with the discovery that surface atomic oxygen species were largely desorbed by this point.

4.13.4 Effect of Reaction Temperature As the reaction temperature is raised from 200-700 °C the conversion of methanol increases along with the production of formaldehyde

88

. An increasing rate of the side reaction to

form carbon monoxide is also observed due to gaseous decomposition of formaldehyde. Carbon dioxide is also formed, with relatively more at temperatures less than 350 oC compared to when temperatures typical of industrial formaldehyde synthesis are employed 88

. The maximum yield of formaldehyde lies at the temperature at which the sum of the

losses due to unreacted methanol and from the formation of side products is at minimum. Temperature is normally controlled by adjusting the amount of air that is supplied.

5.

Polycrystalline Silver Catalyst

5.1

Pure Silver Catalyst

Polycrystalline silver catalyst is typically made by metal refiners. Examination of various commercially available silver catalysts has revealed that they are not the same in terms of physical properties [Figure 15]. This latter observation is at least partially due to the fact that formaldehyde producers usually only request that the silver is at least 99.99 % pure and of specified mesh fractions, hence there is often insufficient quality control in terms of silver catalyst synthesis as desirable properties for formaldehyde synthesis are not explicitly part of the customer specification.



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High Bulk Density

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Medium Density

Figure 15: SEM images of two types of polycrystalline silver catalyst

The basic properties of importance when considering metallic catalysts include: (1) purity; (2) surface area; (3) packing density; (4) porosity; (5) shape; (6) mechanical strength and (7) thermal conductivity. In China, two types of silver catalyst are used, the first is a high density product termed “crystal silver” which is similar to the high bulk density material illustrated in Figure 14. The second material is termed “foam silver” and this has an almost wire like appearance and is characterized by a low bulk density (ca. 1 g/mL). Although the “foam silver” has the highest surface area its performance is hindered by the exceptionally low mechanical stability which leads to limited lifetimes in the order of only a few weeks.

Silver gauze was one of the original catalysts used for formaldehyde synthesis, but it is rarely if ever used today. Nevertheless, recently Baltes et al.

90

described a silver gauze catalyst

which exhibited improved formaldehyde yields which correlated with decreasing wire diameter. Importantly the formaldehyde selectivity was said to increase from 87 to 90 % when changing from a polycrystalline silver catalyst to a wire mesh silver material. Methanol conversion in each instance was 99 %. It was noted however that no catalyst lifetime information was provided and as such it is not known if the mechanical stability of the silver mesh was sufficient for industrial implementation.

5.2

Modified Silver Catalyst

Several efforts to promote the catalytic performance of silver catalyst for formaldehyde synthesis have been described. Claus et al.

91

proposed that silver catalyst could be doped

with Pd, Pt, Rh, Ru and Ir in addition to oxides such as alumina which were stable at


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elevated reaction temperatures of up to 720 oC. An example of a 2 wt% doped silver catalyst was presented wherein the formaldehyde yield was claimed to be 90.4 %. Esumi 92 suggested that silver alloyed with either platinum, palladium or rhodium may be suitable methanol oxidation catalysts. Silver-lead catalyst has also been revealed by Gerberich

93

and under conditions where incomplete methanol conversion was achieved the formaldehyde selectivity was reported as 91.8 %. Silver catalysts modified by the addition of barium, strontium, calcium, and/or indium may be beneficial for oxidation reactions

94

.

BASF promoted the idea of doping the surface of silver catalyst with phosphate species 95-97. Diercks et al. 96 described the application of Na2P2O7 species on silver catalyst and proposed that the activation time was shortened during catalyst light-off conditions. Supported catalysts have also been proposed with silver supported on pumice stone widely reported to have been used in the Russian formaldehyde industry

98

.

Chen et al.

99

proposed that silver coated on ceramic microspheres was an effective catalyst for methanol oxidation to formaldehyde with methanol conversion stated to be in the range 90 to 98 %. Diem et al.

100

similarly indicated that silver deposited on porcelain spheres could be

employed in formaldehyde synthesis reactions.

In another patent, Diem et al.

101

demonstrated that silver particles on an alumina support could also be considered for methanol oxidation purposes.

5.3

Electrochemical Synthesis of Polycrystalline Silver

Two electrochemical cell configurations are employed to manufacture polycrystalline silver catalyst [Figure 16]. A Balbach-Thum cell consists of horizontal cast silver anodes, which are located in a top basket lined with cloth that is designed to collect anode slime and thus prevent contamination of produced silver crystals. In the cell bottom is usually either a carbon or more routinely a stainless steel cathode plate onto which the silver dendrites are deposited. The electrolyte is typically prepared by dissolution of silver in concentrated nitric acid and ultimately may contain from 10 to 200 g/L of silver following dilution with RO water. Other metal ions are present in the electrolyte as a result of migration from the anode material and species may include copper, lead, bismuth, and cadmium. Removal of the product silver crystals is achieved by manual scraping of the cathode plate.



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Balbach-Thum Cell Moebius Cell

Figure 16: Balbach-Thum and Moebius cells for electrocrystallization Moebius first patented a process in 1884 using silver nitrate as a conducting media for the electrolytic production of silver crystals. In the Moebius system, the anodes are cast, drilled and bolted to hanger bars before being immersed in the electrolyte. A woven cloth bag of controlled porosity is carefully placed about the anodes to catch any anode slime produced. Normally, mechanically operated scrapers move back and forth over the cathode surface and thus dislodge silver crystals which then fall to the bottom of the cell and can subsequently be periodically removed.

In general, polycrystalline silver can be obtained by operation of an electrochemical cell, for example the conventional Moebius or Balbach-Thum cells [Figure 16], using a silver nitrate electrolyte in the pH range 1-4 containing between 5 and 100 g/L dissolved silver, a cell temperature of 10-80 oC, a current density between 100 and 3000 A/m2 and a cell voltage between 0.2 and 9 volts

102

. In particular, preparation methods specifically related to

polycrystalline silver catalysts include the continuous electrolytic refining of silver in an aqueous solution of silver nitrate and nitric acid at 24 oC, 3.1 volts and a current density of 1.2 amp/dm2

103

wherein silver grains of 0.2 to 2.5 mm in size are stripped from a slowly

rotating polypropylene anode. Also known is the addition of organic inhibitors to the electrochemical cell to modify the structure of the silver crystals deposited. For example, the addition of thiourea at the solubility limit produces crystals of the unorientated dispersion type 104.


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5.4

Silver Catalyst Bed Design

Silver catalyst beds are typically composed of a range of mesh fractions with the general rule being that the largest granules are placed on the bottom of the bed, medium grains in the middle section and fine silver on the top of the bed. Bed depths range from 1.5 to 3 cm for the majority of catalyst beds in operation today. It is claimed that greater yields can be achieved through use of several layers of catalyst, characterised by differing grain size

70

.

For example, a double layer catalyst comprising both coarse and fine grains has been recommended

105

. Aicher et al.

24

explored the concept of grain size distribution and bed

layer thickness and suggested three or more layers should be employed and specified an overall total layer thickness of 15 to 25 mm. Laying of silver grains only at the outer edge of the silver bed has also been described, presumably to minimize the possibility of the a portion of the feed bypassing the catalyst due to inefficient sealing against the reactor wall 24

. Szustakowski 85 determined that smaller silver catalyst particles enhanced the degree of

methanol conversion and formaldehyde yield, an observation which may relate to the increased silver surface area available for reaction to occur. However, it has also been shown that beds with predominantly large crystals (80 to 90 % from 0.75 to 3 mm crystals) relative to beds containing smaller silver granules (0.2 to 1 mm) gave a significantly higher yield (90.8 % compared to 86.8 %) 77.

Dual bed designs incorporating two different catalysts have also been claimed to be advantageous. Anita

106

suggested that placing a metal oxide catalyst (iron molybdenum

oxide) on top of a silver catalyst can promote formaldehyde synthesis. Zhang et al.

107

proposed that the upper layer of the bed should be silver catalyst and that the lower portion should be comprised of copper granules. The preferred ratio of copper to silver ranged from 5:1 to 1:1. Wachs and Wang

108

also suggested a similar arrangement wherein the

upper layer was silver and the lower layer was copper.

6.

Mechanism for Formaldehyde Synthesis

It is beyond the scope of this article to review the fundamental catalytic science studies in fine detail, however, a brief summation of the main features will be presented here.



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6.1

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Oxygen Chemistry

The oxygen chemistry with silver catalysts is critical in terms of the industrial partial oxidation of methanol to form formaldehyde. Rocha et al.

109

used in situ XPS to examine

the interaction of oxygen with either silver foil, silver powder or single crystals. Five distinct oxygen species were detected and denoted Oα1, Oα2, Oα3, Oβ and Oϒ: Oα1 is ascribed to the formation of oxygen induced reconstruction of crystal faces on silver; Oα2 is weaker in oxidic character than Oα1 and is proposed to be an oxide located at steps and edges on the silver surface; Oα3 is an “electrophilic oxygen” as described by Buhktyiarov and co-workers; Oβ is Oϒ is stable at high

assigned to oxygen atoms located in the bulk silver structure;

temperatures similar to those encountered during industrial methanol oxidation processes, and is assumed to be the active site for formaldehyde synthesis

110

. A range of surface

science investigations have been completed using various silver single crystals to determine how oxygen interacts under different conditions.

Ertl and co-workers [116, 117]

demonstrated that the dominant oxygen species present at industrial conditions were those located in subsurface locations. Incorporation of oxygen species in the silver catalyst promoted the formation of a distinct terrace and facets along with a series of small pits in the surface 111. Scanning Tunnelling Microscopy (STM) images revealed that the Oϒ species was located in the uppermost silver surface layer and this was due to the similar radius of atomic oxygen and metallic silver

112

. Raman spectroscopy has also been used to identify

the molecular and atomic oxygen species present on silver catalyst

84, 113, 114

. The surface

embedded Oϒ species was characterized by a vibration at ca. 810 cm-1 and dissolved atomic oxygen by a band at ca. 610 to 640 cm-1 115. Temperature Programmed Desorption (TPD) of oxygen species present on polycrystalline silver as a function of various heat treatments has shown that Oϒ was stable until at least 600 oC, whereas dissolved atomic oxygen (Oβ) desorbed at ca. 380 oC 116.

6.2

Methanol Oxidation

Contact of only gaseous methanol with silver catalyst does not result in discernible interaction with the surface, but the presence of adsorbed atomic oxygen was demonstrated by Wachs and Madix

117

to promote chemisorption and subsequent reaction to create methoxy,

formaldehyde and formate species. Isotopic labelling revealed that the initial step was the interaction of methanol with atomically adsorbed oxygen to produce a methoxy species.


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Subsequent loss of hydrogen produced formaldehyde and also potentially formate species which were thought to be an intermediate in the combustion process to make carbon dioxide and water. The basic steps in the catalytic reaction are shown below which agree with the work of Wachs & Madix 117.

The Partial Oxidation of Methanol to Formaldehyde Equation 12:

(Y) + () → () + ()

Equation 13:

(Y) + () → () + ()

Equation 14:

() → (Y)

Equation 15:

() → () + ()

Equation 16:

() → (Y)

Carbon Dioxide Production Equation 17:

() + () → () + ()

Equation 18:

() → (Y) + ()

Equation 19:

2 () + () → (Y)

Methyl Formate Equation 20:

() + () → ()

Equation 21:

() → () + ()

Equation 22:

H () → (Y )

Formic acid is normally produced at ppm levels in the silver catalyst process [Equation 22] and excessive amounts can cause problems relating to storage of product formaldehyde.

Equation 23:

+ →

Other undesirable reactions, which must be avoided by proper control of temperature and other factors if high yield are to be obtained, include the previously mentioned thermal decomposition of formaldehyde to form carbon monoxide and hydrogen. The microkinetic modelling of Andreasen and co-workers

118, 119

supported the findings of Wachs and Madix



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117

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. Practically, the operating temperatures for methanol oxidation are commonly in the range

550 to 700 oC and thus the coverage of atomically adsorbed oxygen under these conditions has been suggested to be relatively small 89. Hence, it has been argued that embedded atomic oxygen (Oϒ) is the active species for direct dehydrogenation of methanol to formaldehyde 87, 89. Sun et al. 120 applied density functional theory and found that the OH bond formed by reaction of embedded atomic oxygen (Oϒ) with methanol was weaker than the corresponding OH bond created by interaction of methanol with surface atomic oxygen. As a result, the propensity to form hydrogen was enhanced and thus promoted the dehydrogenation pathway for formaldehyde synthesis.

Temporal-Analysis-of-Products research by van Veen et al.

121

regarding methanol oxidation on polycrystalline silver also supported the idea that embedded atomic oxygen (Oϒ) was indeed highly selective to formaldehyde and hydrogen, whereas surface atomic oxygen was able to oxidise methanol to form carbon dioxide and water. Schubert et al. 112, 122 reported that the reaction with surface oxygen was of limited selectivity, a point also emphasised by Lefferts et al. 123 who indicated that weakly bound oxygen was the cause of methanol combustion. As such the following reactions may apply [Equations 23 to 27].

Equation 24:

(>) + (>]4) → (>) + (>]4)

Equation 25:

(>) → (>) + (>)

Equation 26:

(>]4 ) + (>]4) → (>]4) + (>]4)

Equation 27:

(>) + (>]4) → (>]4)

Equation 28:

(>]4) → (Y ) + "_5 ℎ "

However, Andreasen et al. 118 argued that their microkinetic model for formaldehyde synthesis did not require the assumption of more than one active oxygen species. Instead they indicated that selectivity was related to kinetics and not thermodynamics. At higher temperatures such as those encountered for industrial formaldehyde synthesis it was proposed that consumption of oxygen for formaldehyde production resulted in unavailability of oxygen to cause unwanted side reactions.


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Regardless of the debate regarding the fundamental mechanism for formaldehyde synthesis, “pin hole” defects have been widely reported to be apparent on the silver surface during methanol oxidation conditions in accord with Equation 27 110, 124. For example, Millar et al. 18 used Environmental Scanning Electron Microscopy (ESEM) to observe the formation of “hilllike” structures as the reaction temperature was elevated to ca. 450 oC before widespread production of “pin-holes” at temperatures above ca. 600 oC. Closer inspection of the pin-hole defects using Field Emission Scanning Electron Microscopy revealed the presence of debris comprised of small silver particles in the immediate vicinity of each pin-hole thus confirming an “explosion” of water vapour from the subsurface region of the catalyst occurred 114.

7.

Catalyst Poisons

Industrially, one of the most critical issues with relation to catalyst performance is poisoning of the silver surface which can result in diminished methanol conversion, reduction in selectivity to formaldehyde, enhanced sintering of silver crystals and blockage of the catalyst bed. Inherently, polycrystalline silver catalyst is of low surface area and thus it is particularly susceptible to the effects of even low concentrations of impurities in the feed stream.

7.1

Rust

Catalytic performance of silver can be compromised when iron is present on the surface 125, 126

. Iron (rust) contamination can arise from degradation of plant components and also be

present in feed materials. Deng et al.

125

deposited iron nitrate on a polycrystalline silver

surface and investigated the impact by a variety of surface science techniques. As the extent of iron present on the silver surface was raised, the formaldehyde selectivity decreased and the methanol conversion increased. Consequently it was inferred that the iron species promoted the combustion of methanol. X-ray photoelectron spectroscopy revealed that the oxidation state of the iron species increased as the amount of iron present was enhanced and this behaviour corresponded with increasing production of carbon monoxide and subsequently carbon dioxide during reaction conditions. Jede et al.

127

further illuminated the situation with regards to iron poisoning of silver catalyst by determining that even a 2 % monolayer coverage of the catalyst surface with iron species was sufficient to promote combustion of methanol.



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7.2

Silica

Silica is usually introduced as a catalyst poison through the feed water supply. Without a water purification process the silver catalyst can become spoiled by the deposition of silica on the silver surface. As a result the formaldehyde yield can decrease due primarily to a reduction in methanol conversion. Figure 17 shows a Scanning Electron Microscope (SEM) image of silica deposits on a silver catalyst as confirmed by energy dispersive spectroscopy.

(a)

(b)

Figure 17: Example of silica deposition on a silver catalyst surface (a) SEM image of silver catalyst grain (b) EDS trace for observed impurities

7.3

Carbon

Carbon is almost always found either in the used catalyst bed or in the area immediately below the catalyst bed [Figure 18].

One pathway to carbon formation is via the

decomposition of formaldehyde gas. Equation 29:

(Y) ↔ (Y) + (Y)

The decomposition of formaldehyde is favoured at the high reaction temperatures normally encountered and the extent of this reaction depends upon the residence time of the hot formaldehyde gas in the bed.

The Boudouard reaction can then occur to produce

carbonaceous deposits. Equation 30:

2 ↔ +


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In some instances, carbon is observed as a distinct layer in the silver catalyst bed cross section [Figure 18] directly below an uppermost “white layer” of silver. This behaviour may be due to issues with the feed ratios employed which may accelerate carbon deposition.

Figure 18: Examples of carbon decomposition in a silver catalyst bed

8.

Future Directions

It is clear from this review that the formaldehyde synthesis process using silver catalyst represents a fascinating challenge to improve performance. Key issues which should be addressed include: (1) process optimization to maximise formaldehyde yield and further reduce utilities consumption; (2) development of improved silver catalysts which are tailored specifically for reaction conditions; (3) elimination of poisoning of silver catalysts; (4) creation of a detailed economic model for formaldehyde production with silver catalysts and comparison to the alternate metal oxide process.



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Efforts will still be directed towards development of synthesising formaldehyde directly from methane

128

or carbon dioxide

129

, but it should be remembered that a substantial

amount of capital has been invested in formaldehyde infrastructure hence any substantial changes in the formaldehyde synthesis industry may take many years to appear. As such it is more prospective to optimise current techniques such that the payback to industry could be obtained immediately.

There exists a threat to the formaldehyde industry in terms of the development of nonformaldehyde based resins 130. As this latter market is the major consumer of formaldehyde in many global locations, it would be prudent for the formaldehyde industry to attempt to diversify the uses for formaldehyde. One such example is the oxidation of formaldehyde to formic acid which is a catalytic process which can be added to existing formaldehyde production facilities 131.

9.

Conclusions

Formaldehyde synthesis from methanol using a silver catalyst is a well-established technology that continues to be widely employed industrially, with its continual growth in popularity greatly attributed to the dominance of this process in the Chinese market. There are three commonly employed process variations with the off-gas recycle process variant considered state of the art technology due to reported higher formaldehyde yields and production of high strength formaldehyde concentrations. However, there is no consensus as to the best design of formaldehyde plants with considerable differences in equipment specifications apparent.

Operating conditions are somewhat varied and there is a distinct need to understand the fundamentals of formaldehyde synthesis to a greater degree if enhancement of formaldehyde yields to levels which are comparable with the metal oxide catalysed methanol oxidation process is to be gained.

Academic literature has focussed on

fundamental studies of catalyst mechanisms on silver catalyst but only rarely addressed problems which are industrially important such as catalyst bed design, design of improved catalysts which are industrially viable, impact of catalyst poisoning and how to mitigate these effects, and optimization of process design. It is generally agreed that high space


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velocities are required to minimise side reactions and that reaction temperature should be maintained as low as possible.

Polycrystalline silver catalyst made electrochemically dominates the formaldehyde industry but it is evident that the quality is not consistent and that physical parameters such as high surface area, good porosity, excellent mechanical strength, acceptable thermal conductivity, and a medium bulk density are not always adhered to.

The major drawback of

polycrystalline silver catalysts is their extreme sensitivity to catalyst poisons due to the inherently low surface area present in metallic catalysts. A means to protect the silver catalyst surface from contamination is desirable.

All too often, poor formaldehyde yields or unsatisfactory catalyst lifetimes have been ascribed to undesirable catalyst properties despite the fact no catalyst autopsy was performed or investigation of plant operation completed.

Consequently, there exists

considerable scope to introduce and apply state of the art catalyst science and standardised engineering methods to the formaldehyde industry.



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10.

Acknowledgements

We would like to thank all the formaldehyde producers, formaldehyde equipment suppliers, and personnel who have shared their knowledge with us and allowed us to gain an insight into the fascinating world of industrial methanol oxidation over silver catalyst.


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11.

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Industrial Production of Formaldehyde Using Polycrystalline Silver

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