Spinning Disc Reactor for Green Processing and Synthesis - Process Intensification for Green Chemistry - page 76

_

BF

H H H + BF 3 H

3

O O O O

H H

Si Si Si Si

O O

(a)

BF 3 .(H 22 O)

Si Si

_

BF 2 H

BF 2 H +

O O

O O

Si Si

Si Si

(b)

Scheme 3.1 Grafting of BF3 on silica, yielding two possible structures (a) and (b) for Lewis
acid active centres. Reprinted from [ref 9] # 2006 Wiley Periodicals, Inc.

environment and possible reuse of the catalyst [46]. Through the use of such supported
catalysts, the elimination of the water-quenching step required in a homogeneous system at
the end of the reaction to neutralize the Lewis acid also reduces the production of
hazardous acidic waste.
In order to address these issues, a study involving a performance comparison between
the SDR and a conventional STR for the heterogeneous cationic polymerization of styrene
was undertaken by Boodhoo et al. [9]. A newly developed silica-supported BF 3 Lewis acid
catalyst [47] was used to initiate the polymerization, as illustrated in reaction Schemes 3.1
and 3.2. A summary of the comparison is shown in Table 3.4. It was found that the
polymerization in the batch at about 50% w/w [St] was difficult to control at the operating
temperature of 40 C (Run B8). The reaction spiralled out of control, giving a maximum
temperature of 97 C within 1 minute into the reaction, with an accompanying detrimental
effect on the molecular weight and polydispersity of the polymer formed. These findings
highlight the importance of cooling the batch to very low temperatures despite low rates of
polymerizations, as seen in Run B4. For safe operation in the STR at 40 C, it was
recommended that the maximum monomer concentration be no greater than 25% w/w. On
the other hand, controlled operation at the elevated temperature of 40 C using 49% w/w
[St] was possible in the SDR (Run S14), giving a higher average rate of polymerization and
higher molecular weights than the cooled batch at 13 C (Run B4). More interestingly, the
SDR was demonstrated to be capable of handling styrene concentrations up to 75% w/w in
a safe and controllable manner, with good control of MWDs (Runs S21, S26, S31), at 20
and 40 C disc temperature.

+

CH

CH 2 CH 3 CH

H +

Scheme 3.2 Proton attack of a double bond on a styrene molecule during the initiation step.
Reprinted from [ref 9] # 2006 Wiley Periodicals Inc.