Potential Future Solutions for PO Synthesis Gas Phase Hydrooxidation of Propene with Oxygen and Hydrogen HOPO

The hydro-oxidation reaction involves one mole of propene, one mole of oxygen and one mole of hydrogen, forming PO and water:

6.2 PO-only Routes: Several Approaches for Sustainable Alternatives | 351 O

Competing side reactions include propene hydrogenation to propane, and propene combustion to carbon dioxide and water. Additional hydrogen and oxygen consumption occurs to form water.

The best systems for this reaction are based on Au0 nanoparticles supported either over TiO2 (anatase), or over microporous or mesoporous Ti-silicates (TS-1, Ti-MCM41, Ti-p, Ti-SiO2, Ti-TUD, silylated titanosilicate). The pioneer in this field was Haruta [36]; many studies concerning novel procedures for the preparation of Au-based systems have since been reported in the literature, including various types of supports [37a,l] and the use of other metal active components, for example, Ag, Pd or Pt [26c, 37m,n].

The best results reported are: 0.090-0.120 gPO gcat_1 h_1 productivity, at a propene conversion close to 8%, PO selectivity above 90% and selectivity based on hydrogen over 20%. Table 6.7 shows several results obtained from the literature. A commercially viable process would probably require a propene conversion >10%, PO selectivity >90% based on propene, and >50% based on hydrogen [36v].

Table 6.7 Performance of Au-based catalytic systems in HOPOa.

Catalyst T (°C) C3H6/H2 Selectivity Reference conversion (%) for PO (%)

0.98% Au/TiO2 (anatase)

50

1.1/3.2

>99

[36d]

0.20% Au/TiO2(3%)-SiO2

120

2.5/2.6

93

[36d]

1.0% Au/Ti-MCM-41

100

3.1/47

92

[36p]

1%CsCl-1%Au/Ti-MCM-41

100

1.7/4.5

97

[36p]

1.2% Au/Ti-MCM41

100

1.8/38

95

[36l]

1.0% Au/TiO2-SiO2

100

1.5/13

94

[36l]

1% Au/TiO2

70

1.3/ng

>99

[37b]

1% Au/TS-1

125

0.8/ng

>99

[37b]

1% Au/TS-1

200

2.0/ng

50

[37b]

1% Au/TiO2-SiO2

100

1.0/ng

>99

[37b]

0.37%Au/TiO2(1%)-SiO2

150

6.8/36.4

91

[36s]

0.49% Au/TiO2

150

2.4/44.1

38.6

[36s]

0.06% Au/0.09% Na/TiO2(1%)-SiO2

180

1.5/5.1

96

[36s]

Au/0.025% Mg/TiO2(1%)-SiO2

180

7.3/16.8

92.6

[36s]

Au/TiO2(1%)-SiO2

180

5.8/15.9

94.4

[36s]

"Reaction conditions: [36d]: 0.5 g of catalyst, W/F 0.9gsmL x, feed composition inert/oxygen/ hydrogen/propene = 70/10/10/10 (mol.%). With 0.20 wt% Au/TiO2-SiO2 the catalyst feed composition was 45/10/40/5 (mol.%). [36p]: 0.5 g of catalyst, W/F 0.9gsmL~1, feed composition inert/oxygen/hydrogen/propene = 70/10/10/10 (mol.%). [36l]: 0.5 g of catalyst, W/F 0.9gsmL~1, feed composition inert/oxygen/hydrogen/propene = 70/10/10/10 (mol.%). [37b]: 0.3 g of catalyst, W/F 0.55 gsmL-1, feed composition inert/oxygen/hydrogen/propene = 70/10/10/10 (mol.%). [36s]: 1 g of catalyst, W/F 0.72 g s mL~x, feed composition inert/oxygen/hydrogen/propene = 70/10/ 10/10 (molar ratios); for catalyst 0.06% Au/0.09% Na/TiO2(1%)-SiO2, feed composition inert/ oxygen/hydrogen/propene = 55/5/20/20 (mol.%), W/F 0.45 g s mL^1.

"Reaction conditions: [36d]: 0.5 g of catalyst, W/F 0.9gsmL x, feed composition inert/oxygen/ hydrogen/propene = 70/10/10/10 (mol.%). With 0.20 wt% Au/TiO2-SiO2 the catalyst feed composition was 45/10/40/5 (mol.%). [36p]: 0.5 g of catalyst, W/F 0.9gsmL~1, feed composition inert/oxygen/hydrogen/propene = 70/10/10/10 (mol.%). [36l]: 0.5 g of catalyst, W/F 0.9gsmL~1, feed composition inert/oxygen/hydrogen/propene = 70/10/10/10 (mol.%). [37b]: 0.3 g of catalyst, W/F 0.55 gsmL-1, feed composition inert/oxygen/hydrogen/propene = 70/10/10/10 (mol.%). [36s]: 1 g of catalyst, W/F 0.72 g s mL~x, feed composition inert/oxygen/hydrogen/propene = 70/10/ 10/10 (molar ratios); for catalyst 0.06% Au/0.09% Na/TiO2(1%)-SiO2, feed composition inert/ oxygen/hydrogen/propene = 55/5/20/20 (mol.%), W/F 0.45 g s mL^1.

A particular feature of the Au-based catalyst is the presence of a chemical interaction between titanium and gold, which confers to the system the ability to generate the active species for olefin epoxidation [36]. In fact, the main advantage of these catalysts is that they can epoxidize propene very selectively (selectivities of 95 + %) under mild reaction conditions (50 °C and 1 atm). Higher temperatures are needed when Au is deposited over Ti-silicates, but the latter systems are more stable than those made of Au supported over titanium dioxide, and give a higher PO yield. Modifying the surface of a Ti-Si support by silylation or fluoridation [38] can improve the performance of the catalyst even further.

However, when Au is deposited over other types of supports (silica or alumina), no epoxidation activity is observed [37b]. In addition, neither gold sponge nor titanium dioxide alone are active in the epoxidation of propene; thus, close cooperation between gold and titanium sites exist. For this reason, one major parameter affecting activity is gold dispersion, and hence the contact area between the two components. In fact, the chemistry of preparation of the catalyst, which affects Au dispersion and crystal size, is very important in determining the final catalytic behavior [36q]. Therefore, the use of a support with a surface area optimally higher than 100 m2 g_1 and a low titanium content favor the dispersion of both the Au and Ti4 + active components, allowing optimal cooperation between the two species [36s]. With Au particles between 2 and 5 nm dispersed over titanium dioxide, no dissociation of H2 occurs, and the development of the active species is possible, with good selectivity for PO. With smaller Au particles, hydrogenation of propene to propane becomes the predominant reaction [36r]. The hydrogenation, however, is observed only in the presence of oxygen. This is explained by hypothesizing that oxygen can make the small Au particles electron-deficient, such that Au operates more like a hydrogenating metal (i.e., like Pt or Pd). In contrast, with particles bigger than 10nm, the combustion reaction is favored, because of the reduced Au-Ti interface.

The mechanism proposed involves adsorption of propene onto gold, and the reaction of the adsorbed species with oxygen species (hydroperoxo and peroxo species) formed at the interface between the gold particles and the titanium support, through the reductive activation of oxygen with hydrogen [36j]. Scheme 6.7 shows the reaction mechanism proposed in the literature [37b,g,h].

The rate-determining step is the formation of a peroxide species on Au, whereas the reactive adsorption of propene onto titanium dioxide (catalyzed by the Au

Scheme 6.7 Mechanism of HPPO, with direct synthesis of HOPO. Source: elaborated from [2a].

nanoparticles) to produce a propoxy species on titania is faster. The adsorbed species reacts with the peroxide to generate PO, which desorbs into the gas phase. The O atom left on Ti is consumed by hydrogen dissociatively adsorbed on Au, with the generation of a Ti-OH species and then of water, restoring the Au site. The progressive increase in PO selectivity occurs with a decrease in hydrogen consumption. The fact that gold is relatively unable to dissociatively adsorb molecular oxygen (which would burn hydrogen, and also the hydrocarbons) allows the desired reaction to take place with acceptable selectivity. In general, the hydrogen consumption is high and the selectivity for PO with respect to hydrogen consumed is thus low, at around 10-30%, since most of it is consumed in the production of water. The best hydrogen efficiency reported is no greater than 30%.

Haruta and Oyama proposed a slightly different mechanism [36t, 37s], in which the true intermediate was suggested to be the Ti-hydroperoxo species; the latter is formed by adsorption of O2 onto Au, with the development of a Au + -O2" species, which then reacts with H2 to generate HP. HP finally generates the Ti-OOH active species over tetrahedral Ti sites, for the reaction with propene. Scheme 6.8 summarizes the main steps of the mechanism proposed [36t].

Gas-phase promoters, such as trimethylamine, play an important role in the reaction, adsorbing onto Au particles and preventing hydrogen combustion [36v]. In this case, the deactivation was appreciably depressed with a catalyst made of trimethylsilylated Ba(NO3)2-promoted Au/titanosilicate (Ti/Si 3/100) in the presence of 13-15 ppm trimethylamine as a gaseous promoter. Up to 80% of the high initial activity of the catalyst remained after 5 h of operation (propene conversion roughly 6.5%) with almost constant PO selectivity (about 91%) and H2 efficiency (about 35%), under the following reaction conditions: 150 °C, atmospheric pressure, feed composition C3H6/O2/H2/Ar = 1:1: 1: 7, space velocity (SV) = 4000mLh_1 gcat_1. The PO productivity was approximately 0.08 g gcat_1 h_1, that is to say, not very different from the EO space-time yield in industrial ethene epoxidation.

Scheme 6.8 Mechanism of HOPO, with direct synthesis of HP. Source: elaborated from [36t].

Scheme 6.8 Mechanism of HOPO, with direct synthesis of HP. Source: elaborated from [36t].

Moisture also enhances the catalytic activity by no less than two orders of magnitude [36y]. Water is proposed to play two roles in the reaction, namely in the activation of oxygen and in the decomposition of carbonate. The presence of reducing agents in the reaction environment may also considerably enhance the catalytic performance with regard to both conversion and selectivity. In particular, the preferred gas-phase reductants are CO, NO, 2-propanol and methanol. Improved hydrogen efficiencies can be obtained using CsCl as a promoter [36p], or other alkali or alkaline earth metal ions [39a]. These promoters block acidic sites on the catalyst, which could decompose or oligomerize PO; in general, desorption of PO is favored by an increase in the surface basicity of the catalyst [37i]. Also, alloying the gold with small quantities of platinum greatly improves the hydrogen efficiency [40].

Recently, Bayer reported a promising PO space-time yield of more than 0.2 gPO gcat 1 h_1, with selectivity for PO of 95%, obtained with a propene/oxygen/hydrogen feed of molar composition 30:10:60, at 180 °C and 2 atm pressure [41a]. However, as with the other catalysts, the main problem was a rapid catalyst deactivation. Key issues of Bayer's process include:

1. A catalyst based on Au/(Ag) particles deposited over Ti-containing organic/ inorganic hybrid silicon oxide material, prepared by a sol-gel method. The surface of the catalyst is modified with Si alkyl/alkoxy compounds [41a,c,f,i,k]; therefore, one important feature of the catalyst is its surface hydrophobicity. The material had a DRIFT spectrum characterized by the presence of bands assigned to the hydrocarbon coating (methyl groups), and to Si-H groups. A maximum yield of 10.4% (selectivity 95%) was reported, obtained at 140 °C and 4 atm [41j]. By-products were propionaldehyde (selectivity 2%), acetaldehyde (1%), acetone (<1%), traces of acetic acid, propene glycol and butanedione [41l]. A non-hybrid catalyst, based on nanosized Au particles supported over Ti-containing support, gave a PO yield of 5.8% at 46 °C and feed composition of propene/oxygen/ hydrogen 0.4:0.1:1.3 (molar ratios) [41e].

2. A method for the regeneration of spent catalysts, that includes placing them in contact with water and a diluted hydroperoxide solution [41b].

3. The use of CO as a gas-phase promoter, to limit catalyst deactivation; for example, with a catalyst made of Au with a particle size less than 4 nm, deposited over titanium dioxide, a yield for PO of 1.4% at 10 °C (selectivity >99%) was maintained over 6 h reaction time, whereas in the absence of CO the yield declined over the same period of time [41d].

4. The use of additional catalyst components, for example, Mo6 +, as promoters for Au-based catalysts [41g].

The use of a hybrid TS-1, that is to say, a TS-1 in which non-hydrolysable organic ligands have been incorporated while the crystalline structure of TS-1 is largely retained, has also been reported for the epoxidation of propene with aqueous HP [41h]. This structure is reported to possess enhanced hydrophobic properties, despite the relatively large amount of Ti incorporated; this property enhances the adsorption of propene and the desorption of PO, thus limiting the consecutive reaction on PO, and limiting the decomposition of HP.

Both Pd- and Pt-based systems are very active in propene and hydrogen conversion, but both catalyze the hydrogenation of propene. Nevertheless, an outstanding PO yield of 11.7% (selectivity 46%) was obtained on a TS-1 catalyst loaded with 1 wt% Pd and 0.02 wt% Pt, autoreduced under N2 flow at 150 °C. The increase in PO yield by the addition of minor amounts of Pt to a Pd/TS-1 catalyst was correlated with an increase in the fraction of Pd2 + species, which play an important role in the reaction mechanism [26c].

This reaction has also been thoroughly investigated by Dow Chem Co [39]. For example, the presence of Na/Mg in Au/TiO2 systems allows the catalyst surface to be kept cleaner, thus increasing the turnover of catalytic sites and also decreasing the rate of deactivation [39b]. The selectivity with respect to the hydrogen consumed is low, as indicated by the high H2O/PO ratio. In most of the Dow patents, propene conversions lower than 1% are reported, selectivity for PO is well above 90%, the H2O/PO ratio is between 3 and 10, with catalysts based on Au supported over various Ti-containing supports (TS-1, TS-2, Ti-MCM, Ti-b, Ti grafted over silica, etc.), and doped with alkali metal ions, alkaline earth and eventually lanthanides. However, in one patent a propene conversion of 3.4% was attained, with selectivity for PO of 97% [39j], at 140 °C, atmospheric pressure, with a feed composed of 20% propene, 10% oxygen, 10% hydrogen, and overall GHSV 960h_1. The catalyst was made of Ba/Na-doped Au (average particle size of 5 nm) supported over a Ti-silicate, with 1.5% highly dispersed Ti.

Besides supported Au [39f,i,l,o], other catalysts claimed in Dow patents include Ag/ alkali (alkaline earths, lanthanides) over Ti-containing supports. These systems give a propene conversion lower than 1%, high PO selectivity (but lower than that achieved with Au-based systems) and a H2O/PO ratio much higher than that obtained with Au-based systems [39c,d,h,m,n]. The co-presence of Au, however, remarkably reduces the H2O/PO ratio, thus increasing hydrogen efficiency [39g,k].

UOP has investigated the synthesis of HP by a reaction between hydrogen and oxygen with a favorable molar ratio close to 1:1, at 30 atm, without inert gases, at high space-time yield and in the presence of a heterogeneous catalyst, inside a micro-structured apparatus, which guarantees safe plant operation within the explosive regime. UOP's interest is to have this direct HP synthesis route in the framework of PO manufacture [42]. Microchannel reactors are intrinsically safe, because thermal runaway and uncontrolled self-accelerating radical-chain propagation do not occur therein. In fact, the flame arrestor effect that quenches chain-growth by chain termination at the channel walls is favored because of the large interfaces and short diffusion distances. A space-time yield of 2 gHP gcat_1 h_1 is claimed, which is considerably higher than that achieved with conventional reactors.

Integration of the processing of propene epoxidation with that of propane dehy-drogenation should in principle lead to economic advantages [36s]. In the integrated process, oxygen is added at the outlet from the dehydrogenation reactor, in the presence of an Au-based catalyst, to produce a gas that contains PO, unreacted propane, propene, hydrogen and oxygen. The epoxide is separated from the other gases. Before recycling the remaining gas to the dehydrogenation reactor, oxygen is eliminated from the gas mixture by letting it react with part of the hydrogen. In this way, hydrogen does not accumulate in the recycling loop. The advantage of this process configuration is that there is no need to separate hydrogen from the other gases.

Catalyst deactivation is one problem in the systems described in the literature. Deactivation is caused by the consecutive oxidation of the propoxy species to carboxylates, and to oligomerization of PO with accumulation of oligomerized and oxidized PO by-products around the gold nanoparticles. Other major hurdles for the industrial application of this process are the low propene conversion, the lower activity of the successively regenerated catalyst, and, especially, the low H2 efficiency. A higher hydrogen selectivity will be needed for better process economics to avoid excessive costs for hydrogen feed.

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